Assessing cancer risks from short-term exposures in children

A population-based study of the epidemiology of oral squamous cell carcinoma in the Netherlands 1989-2018, with emphasis on young adults

There has been an increasing trend in oral squamous cell carcinoma (OSCC) in patients under 45 years of age. The aim of this study was to evaluate the burden of OSCC in the Netherlands between 1989 and 2018 among young adults (age 20-34 years) when compared to adults (age 35-44 years), and to describe the burden in older groups as well, utilizing cancer registry data to characterize incidence patterns by age, sex, and risk factors. A total of 18,963 cases of OSCC were reported. The overall incidence rate, as measured by annual percentage change (APC), increased significantly from 1989 to 2010 by 1.3% per year (95% confidence interval (CI) 0.9-1.7%) but decreased thereafter by -0.9% (95% CI -2.5% to 0.7%). Annual incidence increased significantly by 2.4% (95% CI 1.1-3.8%) for patients aged 20-34 years, while it decreased for those aged 35-44 years by -0.9% (95% CI -1.7% to 0.0%). In patients older than 60 years, incidence rates increased overall (60-74 years: APC 1.8%, 95% CI 1.5-2.1%; ≥75 years: APC 1.5%, 95% CI 1.2-1.9%). Overall, 66.5% of patients were smokers and 65.3% were alcohol consumers. The marked differences in incidence within the young age subgroups warrants further investigation to elucidate any likely disparity in biological process and clinical outcomes in these populations.

Spatial, temporal variability and carcinogenic health risk assessment of nitrosamines in a drinking water system in China

Nitrosamines, as a class of emerging frequently detected nitrogenous disinfection byproducts (N-DBPs) in drinking water, have gained increasing attention due to their potentially high health risk. Few studies focus on the occurrence variation and carcinogenic health risk of nitrosamines in drinking water systems. Our study aimed to investigate the spatial and temporal variability of nitrosamines in a drinking water system and to conduct a carcinogenic health risk assessment. Three types of water samples, including influent water, treated water and tap water, were collected monthly during an entire year in a drinking water system utilizing a combination of chlorine dioxide and chlorine in central China, and 9 nitrosamines were measured. The nitrosamine formation potentials (FPs) in influent water were also determined. N-nitrosodimethylamine (NDMA) was the most prevalent compound and was dominant in the water samples with average concentrations ranging from 2.5 to 67.4 ng/L, followed by N-nitrosodiethylamine (NDEA) and N-nitrosopiperidine (NPIP). Nitrosamine occurrence varied monthly, and significant seasonal differences were observed in tap water (p < .05). There were decreasing mean NDMA, NDEA and NPIP concentrations from influent water to treated water to tap water, but no significant spatial variability was observed within the water distribution system (p > .05). The average and 95th percentile total lifetime cancer risks for the three main nitrosamines were 4.83 × 10^-5 and 4.48 × 10^-4, respectively, exceeding the negligible risk level (10^-6) proposed by the USEPA. Exposure to nitrosamines in drinking water posed a higher cancer risk for children than for adults, and children aged 0.75 to 1 years suffered the highest cancer risk. These results suggest that nitrosamine occurrence in tap water varied temporally but not spatially. Exposure to drinking water nitrosamines may pose a carcinogenic risk to human health, especially to children.

PM2.5-bound polycyclic aromatic hydrocarbons and nitro-polycyclic aromatic hydrocarbons inside and outside a primary school classroom in Beijing: Concentration, composition, and inhalation cancer risk

PM_2.5 samples were collected inside and outside a primary school classroom in Beijing in 2015 and analysed for 11 polycyclic aromatic hydrocarbons (PAHs) and 10 nitro-PAHs (NPAHs). In the sampling period in the heating season (namely, the heating period), the median concentrations of indoor and outdoor PAHs were 223 ng/m³ and 264 ng/m³, respectively, and those of indoor and outdoor NPAHs were 3.61 ng/m³ and 5.12 ng/m³, respectively. The concentrations of PAHs and NPAHs were consistently higher in the heating period than those (indoor PAHs: 8.75 ng/m³, outdoor PAHs: 8.95 ng/m³, indoor NPAHs: 0.25 ng/m³, outdoor NPAHs: 0.40 ng/m³) in the sampling period in the non-heating season (namely, the non-heating period). In both periods, total PAHs and total NPAHs in indoor PM_2.5, as well as most individual PAHs and NPAHs, were positively correlated with the outdoor PAH and NPAH concentrations (p < 0.05). This finding suggests that indoor PAHs and NPAHs are largely dependent on outdoor inputs. It is inferred from the diagnostic ratios that PAHs and NPAHs in indoor and outdoor PM_2.5 were affected jointly by coal combustion and vehicular emission in the heating period and mainly derived from vehicle exhaust in the non-heating period. Both indoor and outdoor PM_2.5 showed considerable benzo[a]pyrene equivalent toxicity (BaP_eq), especially in the heating period. Benzo[c]fluorene (BcFE) had relatively low concentrations but large contributions to BaP_eq in both periods. This is the first report of PM_2.5-bound BcFE inside and outside classrooms in Beijing. This result indicates that neglecting PAHs with low abundance but high toxicity leads to a significant underestimation of the overall PAH toxicity. The inhalation cancer risk (CR) of PAHs and NPAHs in PM_2.5 during the primary school year exceeded the acceptable level as defined by the U.S. EPA, emphasizing its impact on the lifetime CR in schoolchildren.

Comparison of carcinogenic potency across life stages: implications for the assessment of transplacental cancer risk

Cancer development from in utero exposure has been documented for a variety of agents with the most commonly studied compounds exhibiting mutagenic and genotoxic carcinogen properties. Age-dependent adjustment factors (ADAFs) are applied by the US Environmental Protection Agency to many such carcinogens when assessing cancer risk from early postnatal (PN) exposures; however, this approach has not been widely considered for transplacental (TP) exposure. To explore this question and further evaluate prenatal susceptibility a database of early life animal studies developed by California Office of Environmental Health Hazard Assessment (OEHHA) enhanced with additional literature searching was evaluated. Nine genotoxic carcinogens and one mixture (cigarette smoke) have data available via TP only, PN only and adult-only protocols. Potency comparisons across these lifestages displayed similarly greater potency in male liver and brain compared to adult-only exposure. Both TP and PN exposures were not markedly different than adult-only for other targets such as female liver and blood-borne tumors. Similarity in TP and PN targets and potency for carcinogen action suggests that a 10-fold ADAF may be applied for TP exposure as is currently applied to PN exposure. A similar conclusion was reached by OEHHA. The implications of this heightened TP vulnerability are greatest for less-than-lifetime exposure and this approach might be used to assess the level of cancer risk from exposures during pregnancy. A case example employing a mutagenic flame retardant was used to exemplify application of a TP ADAF for evaluating risks during pregnancy.

Carcinogenic risk of N-Nitrosamines in Shanghai Drinking Water: Indications for the Use of Ozone Pretreatment

N-Nitrosamines are drinking water disinfection byproducts that pose a high carcinogenic risk. We hypothesized that raw water treatment processes influence the types and concentrations of nitrosamines in drinking water, thereby posing differential health risks. We compared the finished water of two water treatment plants (WTP-A, WTP-B) serving Shanghai, China. Both plants use the Qingcaosha reservoir as a water source to generate drinking water with conventional but distinct treatment processes, namely preoxidation with sodium hypochlorite (WTP-A) vs ozone (WTP-B). Average nitrosamine concentrations, especially that of the probable human carcinogen (2A) N-nitrosodimethylamine, were higher in finished (drinking) water from WTP-A (35.83 ng/L) than from WTP-B (5.07 ng/L). Other differences in mean nitrosamines in drinking water included N-nitrosodipropylamine (42.62 ng/L) and N-nitrosomethylethylamine (26.73 ng/L) in WTP-A in contrast to N-nitrosodiethylamine (7.26 ng/L) and N-nitrosopyrrolidine (59.12 ng/L) in WTP-B. The estimated adult cancer risk from exposure to mixed nitrosamines was 1.83 times higher from WTP-A than from WTP-B drinking water. Children exposed to nitrosamines had a significantly higher cancer risk than adults ( p < 0.05). Disease burden exceeded 10⁶ person-years. Taken together, these data suggest that use of ozone in the preoxidation step can reduce nitrosamine formation in drinking water and thereby lower the population cancer health risk.

Metabolomics of Children and Adolescents Exposed to Industrial Carcinogenic Pollutants

Studies on metabolomes of carcinogenic pollutants among children and adolescents are limited. We aim to identify metabolic perturbations in 107 children and adolescents (aged 9-15) exposed to multiple carcinogens in a polluted area surrounding the largest petrochemical complex in Taiwan. We measured urinary concentrations of eight carcinogen exposure biomarkers (heavy metals and polycyclic aromatic hydrocarbons (PAHs) represented by 1-hydroxypyrene), and urinary oxidative stress biomarkers and serum acylcarnitines as biomarkers of early health effects. Serum metabolomics was analyzed using a liquid chromatography mass spectrometry-based method. Pathway analysis and "meet-in-the-middle" approach were applied to identify potential metabolites and biological mechanisms linking carcinogens exposure with early health effects. We found 10 potential metabolites possibly linking increased exposure to IARC group 1 carcinogens (As, Cd, Cr, Ni) and group 2 carcinogens (V, Hg, PAHs) with elevated oxidative stress and deregulated serum acylcarnitines, including inosine monophosphate and adenosine monophosphate (purine metabolism), malic acid and oxoglutaric acid (citrate cycle), carnitine (fatty acid metabolism), and pyroglutamic acid (glutathione metabolism). Purine metabolism was identified as the possible mechanism affected by children and adolescents' exposure to carcinogens. These findings contribute to understanding the health effects of childhood and adolescence exposure to multiple industrial carcinogens during critical periods of development.

Metabolic Disruption Early in Life is Associated With Latent Carcinogenic Activity of Dichloroacetic Acid in Mice

Early-life environmental factors can influence later-life susceptibility to cancer. Recent evidence suggests that metabolic pathways may mediate this type of latency effect. Previously, we reported that short-term exposure to dichloroacetic acid (DCA) increased liver cancer in mice 84 weeks after exposure was stopped. Here, we evaluated time course dynamics for key events related to this effect. This study followed a stop-exposure design in which 28-day-old male B6C3F1 mice were given the following treatments in drinking water for up to 93 weeks: deionized water (dH2O, control); 3.5 g/l DCA continuously; or 3.5 g/l DCA for 4-52 weeks followed by dH2O. Effects were evaluated at eight interim time points. A short-term biomarker study was used to evaluate DCA effects at 6, 15, and 30 days. Liver tumor incidence was higher in all DCA treatment groups, including carcinomas in 82% of mice previously treated with DCA for only 4 weeks. Direct effects of DCA in the short-term study included decreased liver cell proliferation and marked mRNA changes related to mitochondrial dysfunction and altered cell metabolism. However, all observed short-term effects of DCA were ultimately reversible, and prior DCA treatment did not affect liver cell proliferation, apoptosis, necrosis, or DNA sequence variants with age. Key intermediate events resulting from transient DCA exposure do not fit classical cytotoxic, mitogenic, or genotoxic modes of action for carcinogenesis, suggesting a distinct mechanism associated with early-life metabolic disruption.

Latent carcinogenicity of early-life exposure to dichloroacetic acid in mice

Environmental exposures occurring early in life may have an important influence on cancer risk later in life. Here, we investigated carryover effects of dichloroacetic acid (DCA), a small molecule analog of pyruvate with metabolic programming properties, on age-related incidence of liver cancer. The study followed a stop-exposure/promotion design in which 4-week-old male and female B6C3F1 mice received the following treatments: deionized water alone (dH2O, control); dH2O with 0.06% phenobarbital (PB), a mouse liver tumor promoter; or DCA (1.0, 2.0 or 3.5g/l) for 10 weeks followed by dH2O or PB (n = 20-30/group/sex). Pathology and molecular assessments were performed at 98 weeks of age. In the absence of PB, early-life exposure to DCA increased the incidence and number of hepatocellular tumors in male and female mice compared with controls. Significant dose trends were observed in both sexes. At the high dose level, 10 weeks of prior DCA treatment induced comparable effects (≥85% tumor incidence and number) to those seen after continuous lifetime exposure. Prior DCA treatment did not enhance or inhibit the carcinogenic effects of PB, induce persistent liver cytotoxicity or preneoplastic changes on histopathology or alter DNA sequence variant profiles within liver tumors compared with controls. Distinct changes in liver messenger RNA and micro RNA profiles associated with prior DCA treatment were not apparent at 98 weeks. Our findings demonstrate that early-life exposure to DCA may be as carcinogenic as life-long exposures, potentially via epigenetic-mediated effects related to cellular metabolism.

Exposure to chemicals and radiation during childhood and risk for cancer later in life

PURPOSE

Many chemical carcinogens are in food, water, air, household products, and personal care products. Although genetic susceptibility is an important factor in how an individual responds to exposure to a carcinogen, heritable genetic factors alone account for only a minor portion of cancer rates.

METHODS

We review the evidence that early life exposure to carcinogenic chemicals and ionizing radiation results in elevations in cancer later in life.

RESULTS

Because cells are rapidly dividing and organ systems are developing during childhood and adolescence, exposure to carcinogens during these early life stages is a major risk factor for cancer later in life. Because young people have many expected years of life, the clinical manifestations of cancers caused by carcinogens have more time in which to develop during characteristically long latency periods. Many chemical carcinogens persist in the body for decades and increase risk for all types of cancers. Carcinogens may act via mutagenic, nonmutagenic, or epigenetic mechanisms and may also result from disruption of endocrine systems. The problem is magnified by the fact that many chemical carcinogens have become an integral part of our food and water supply and are in air and the general environment.

CONCLUSIONS

The early life onset of a lifelong exposure to mixtures of multiple environmental chemical carcinogens and radiation contributes significantly to the etiology of cancer in later life.

Human disease resulting from exposure to electromagnetic fields

Electromagnetic fields (EMFs) include everything from cosmic rays through visible light to the electric and magnetic fields associated with electricity. While the high frequency fields have sufficient energy to cause cancer, the question of whether there are human health hazards associated with communication radiofrequency (RF) EMFs and those associated with use of electricity remains controversial. The issue is more important than ever given the rapid increase in the use of cell phones and other wireless devices. This review summarizes the evidence stating that excessive exposure to magnetic fields from power lines and other sources of electric current increases the risk of development of some cancers and neurodegenerative diseases, and that excessive exposure to RF radiation increases risk of cancer, male infertility, and neurobehavioral abnormalities. The relative impact of various sources of exposure, the great range of standards for EMF exposure, and the costs of doing nothing are also discussed.

Options for incorporating children's inhaled dose into human health risk assessment

Increasing attention has been placed on inhalation dosimetry in children because of children's greater air intake rate and unique windows of vulnerability for various toxicants and health outcomes. However, risk assessments have not incorporated this information because dosimetric adjustments have focused upon extrapolation across species rather than across age groups within the human population. The objectives of this study were to synthesize information regarding child/adult intake and dosimetry differences for particles and gases for potential application to risk assessment. Data and models gathered at a 2006 workshop and more recent studies were reviewed to better understand lung development and inhaled dose in children. The results show that child/adult differences exist both on a chemical intake basis and on a deposited or systemic dose basis. These differences can persist for several years and are not captured by standard intraspecies uncertainty factors or by USEPA's reference concentration (RfC) methodology. Options for incorporating children's inhalation exposures into human risk assessments include (1) 3-fold default air intake adjustment for the first 3 years of life with a reduced factor for older children; (2) superseding this default via simplified dosimetry models akin to USEPA's RfC methodology modified for children; (3) utilizing more sophisticated models with better anatomical and air flow descriptions; (4) running these models with input distributions to reflect interchild variability; (5) developing more advanced approaches involving imaging techniques and computational fluid dynamic (CFD) models. These options will enable children's inhaled dose to have a quantitative role in risk assessment that has been lacking and will establish a basis for ongoing research.

Electromagnetic fields and cancer: the cost of doing nothing

Everyone is exposed to electromagnetic fields (EMFs) from electricity (extremely low frequency, ELF), communication frequencies, and wireless devices (radiofrequency, RF). Concern of health hazards from EMFs has increased as the use of cell phones and other wireless devices has grown in all segments of society, especially among children. While there has been strong evidence for an association between leukemia and residential or occupational exposure to ELF EMFs for many years, the standards in existence are not sufficiently stringent to protect from an increased risk of cancer. For RF EMFs, standards are set at levels designed to avoid tissue heating, in spite of convincing evidence of adverse biological effects at intensities too low to cause significant heating. Recent studies demonstrate elevations in rates of brain cancer and acoustic neuroma only on the side of the head where individuals used their cell phone. Individuals who begin exposure at younger ages are more vulnerable. These data indicate that the existing standards for radiofrequency exposure are not adequate. While there are many unanswered questions, the cost of doing nothing will result in an increasing number of people, many of them young, developing cancer.

Sequencing analysis of mutations induced by N-ethyl-N-nitrosourea at different sampling times in mouse bone marrow

In our previous study (Wang et al., 2004, Toxicol. Sci. 82: 124-128), we observed that the cII gene mutant frequency (MF) in the bone marrow of Big Blue mice showed significant increase as early as day 1, reached the maximum at day 3 and then decreased to a plateau by day 15 after a single dose of carcinogen N-ethyl-N-nitrosourea (ENU) treatment, which is different from the longer mutation manifestation time and the constancy of MFs after reaching their maximum in some other tissues. To determine the mechanism underlying the quick increase in MF and the peak formation in the mutant manifestation, we examined the mutation frequencies and spectra of the ENU-induced mutants collected from different sampling times in this study. The cII mutants from days 1, 3 and 120 after ENU treatment were randomly selected from different animals. The mutation frequencies were 33, 217, 305 and 144 x 10(-6) for control, days 1, 3, and 120, respectively. The mutation spectra at days 1 and 3 were significantly different from that at day 120. Considering that stem cells are responsible for the ultimate MF plateau (day 120) and transit cells are accountable for the earlier MF induction (days 1 or 3) in mouse bone marrow, we conclude that transit cells are much more sensitive to mutation induction than stem cells in mouse bone marrow, which resulted in the specific mutation manifestation induced by ENU.

Early life and adult exposure to isoflavones and breast cancer risk

Soy and red-clover isoflavones are commonly consumed within the diet or as a dietary supplement due to a range of presumed beneficial health benefits. These isoflavones are thought to protect against heart diseases as well as breast and other types of cancer. Isoflavones are structurally similar to estrogens and may act as estrogen agonists or antagonists by binding to estrogen receptors. Because of an increased use of isoflavones in processed foods and dietary supplements as well as the greater consumption of soy products, dietary intakes of isoflavones are increasing in children and adolescents in North America. Estrogens are a known component of numerous hormone related cancers including breast cancer. It is with these facts in mind that we review the existing epidemiological and experimental animal studies for a resolution to a proposed correlation between increased isoflavone consumption and breast cancer. There is conflicting evidence from epidemiological, intervention and experimental animal studies regarding the chemopreventing effects of soy isoflavones in breast cancer. Isoflavones are weak estrogens and their effect depends upon the dose, time of exposure and species involved. It would, therefore, not be safe to indisputably accept soy or red-clover as a source of isoflavone resource to prevent breast cancer.

Mode of action frameworks: a critical analysis

Mode of action (MOA) information is increasingly being applied in human health risk assessment. The MOA can inform issues such as the relevance of observed effects in laboratory animals to humans, and the variability of response within the human population. Several collaborative groups have developed frameworks for analyzing and utilizing MOA information in human health risk assessment of environmental carcinogens and toxins, including the International Programme on Chemical Safety, International Life Sciences Institute, and U.S. Environmental Protection Agency. With the goal of identifying gaps and opportunities for progress, we critically evaluate several of these MOA frameworks. Despite continued improvement in incorporating biological data in human health risk assessment, several notable challenges remain. These include articulation of the significant role of scientific judgment in establishing an MOA and its relevance to humans. In addition, binary (yes/no) decisions can inappropriately exclude consideration of data that may nonetheless be informative to the overall assessment of risk. Indeed, the frameworks lack a broad consideration of known causes of human disease and the potential for chemical effects to act additively with these as well as endogenous background processes. No integrated analysis of the impact of multiple MOAs over the same dose range, or of varying MOAs at different life stages, is included. Separate consideration of each MOA and outcome limits understanding of how multiple metabolites, modes, and toxicity pathways contribute to the toxicological profile of the chemical. An extension of the analyses across outcomes with common modes is also needed.

Approaches to acrylamide physiologically based toxicokinetic modeling for exploring child-adult dosimetry differences

Dietary exposure to acrylamide is common as a result of its formation during the cooking of carbohydrate foods. This leads to widespread human exposure in adults and children alike. Acrylamide is neurotoxic and is metabolized by cytochrome P-450 (CYP) 2E1 to a mutagenic epoxide, glycidamide. This article describes a modeling framework for assessing acrylamide and glycidamide dosimetry in rats and human adults and children. The challenges in building a physiologically based toxicokinetic (PBTK) model that is compatible with existing rat and human data are described, with an emphasis on calibration against the hemoglobin adduct database. This exploratory PBTK model was adapted to children by incorporating life-stage-specific parameters consistent with children's changing physiology and metabolic capacity for processes involved in acrylamide disposition in terms of CYP2E1, glutathione conjugation, and epoxide hydrolase. Monte Carlo analysis was used to simulate the distribution of internal doses to gain an initial understanding of the range of child/adult differences possible. This analysis suggests modest dosimetry differences between children and adults, with area-under-the-curve (AUC) doses for the 99th percentile child up to fivefold greater than the median adult for both acrylamide and glycidamide. Early life immaturities tended to exert a greater effect on acrylamide than glycidamide dosimetry because immaturities in CYP2E1 and glutathione counteract one another for glycidamide AUC, but both lead to greater acrylamide dose. The analysis points toward glutathione conjugation parameters as being particularly influential and uncertain in early life, making this a key area for future research.

Dose-response relationship for life-shortening and carcinogenesis in mice irradiated at day 7 postnatal age with dose range below 1 Gy of gamma rays

This study was designed to elucidate the dose-response relationships for life-shortening and tumorigenic effect in the dose range below 1 Gy of gamma rays delivered during the infant period. Female B6C3F1 mice were irradiated with 0.10, 0.48 or 0.95 Gy at 7 days of age. All irradiated mice were allowed to live out their entire life span together with a simultaneously ongoing control group under a specific pathogen-free condition. Shortening of the mean life span was 1.58% in mice irradiated with 0.10 Gy, which was statistically significant . The coefficient of the linear dose-response relationship for life-shortening was 11.21% Gy(-1). The attributable death fraction for all causes of death in 0.10 Gy group reached 0.092. The excess relative risk for death rate from all causes was 0.102 in the group irradiated with 0.10 Gy. The coefficient of the linear dose-response relationship of the excess relative risk for death rate from all causes was 1.30 Gy(-1). The mean number of types of solid tumors at the time of death in mice irradiated with 0.10 Gy was distinctly larger than that in the control group. The excess relative risk for death rate from solid tumors was 0.45 in mice irradiated with 0.10 Gy. The coefficient of the linear dose-response relationship of excess relative risk for death rate from solid tumors was 4.52 Gy(-1). Increase in incidences of the pituitary, ovarian and adrenal tumors was observed in mice irradiated with 0.10 Gy. The results of the present study showed that infant mice are susceptible to solid tumor induction, especially of the endocrine organs.

Assessing susceptibility from early-life exposure to carcinogens

Cancer risk assessment methods currently assume that children and adults are equally susceptible to exposure to chemicals. We reviewed available scientific literature to determine whether this was scientifically supported. We identified more than 50 chemicals causing cancer after perinatal exposure. Human data are extremely limited, with radiation exposures showing increased early susceptibility at some tumor sites. Twenty-seven rodent studies for 18 chemicals had sufficient data after postnatal and adult exposures to quantitatively estimate potential increased susceptibility from early-life exposure, calculated as the ratio of juvenile to adult cancer potencies for three study types: acute dosing, repeated dosing, and lifetime dosing. Twelve of the chemicals act through a mutagenic mode of action. For these, the geometric mean ratio was 11 for lifetime exposures and 8.7 for repeat exposures, with a ratio of 10 for these studies combined. The geometric mean ratio for acute studies is 1.5, which was influenced by tissue-specific results [geometric mean ratios for kidney, leukemia, liver, lymph, mammary, nerve, reticular tissue, thymic lymphoma, and uterus/vagina > 1 (range, 1.6-8.1); forestomach, harderian gland, ovaries, and thyroid < 1 (range, 0.033-0.45)]. Chemicals causing cancer through other modes of action indicate some increased susceptibility from postnatal exposure (geometric mean ratio is 3.4 for lifetime exposure, 2.2 for repeat exposure). Early exposures to compounds with endocrine activity sometimes produce different tumors after exposures at different ages. These analyses suggest increased susceptibility to cancer from early-life exposure, particularly for chemicals acting through a mutagenic mode of action.

Age-dependent sensitivity of Big Blue transgenic mice to the mutagenicity of N-ethyl-N-nitrosourea (ENU) in liver

The incidence of childhood cancer is increasing and recent evidence suggests an association between childhood cancer and environmental exposure to genotoxins. In the present study, the Big Blue transgenic mouse model was used to determine whether specific periods in early life represent windows of vulnerability to mutation induction by genotoxins in mouse liver. Groups of mice were treated with single doses of 120 mg N-ethyl-N-nitrosourea (ENU)/kg body weight or the vehicle either transplacentally to the 18-day-old fetus or at postnatal days (PNDs) 1, 8, 15, 42 or 126; the animals were sacrificed 6 weeks after their treatment. The cII mutation assay was performed to determine the mutant frequencies (MFs) in the livers of the mice. Liver cII MFs for both sexes were dependent on the age at which the animals were treated. Perinatal treatment with ENU (either transplacental treatment to the 18-day-old fetus or i.p. injection at PND 1) induced relatively high MFs. However, ENU treatment at PNDs 8 and 15 resulted in the highest mutation induction. The lowest mutation induction occurred in those animals treated as adults (PND 126). For instance, the cII MF for the PND 8 female group was 646 x 10(-6) while the MF for female adults was only 145 x 10(-6), a more than 4-fold difference. Molecular analysis of the mutants found that A:T-->T:A transversions and A:T-->G:C transitions characterized the pattern of mutations induced by ENU in both the neonate and adult mice, while the predominate type of mutation in the controls was G:C-->A:T. The results indicate that mouse liver is most sensitive to ENU-induced mutation during infancy. This period correlates well with the age-dependent sensitivity to carcinogenicity in mouse liver, suggesting that mutation is an important rate-limiting factor for age-related carcinogenesis.

Review and analysis of inhalation dosimetry methods for application to children's risk assessment

Young children have a greater ventilation rate per body weight or pulmonary surface area as compared to adults. The implications of this difference for inhalation dosimetry and children's risk assessment were evaluated in runs of the U.S. Environmental Protection Agency (U.S. EPA) 1994 reference concentration (RfC) methodology and the ICRP 1994 inhalation dosimetry model. Dosimetry estimates were made for 3-mo-old children and adults for particles and Category 1 and 2 reactive gases in the following respiratory-tract regions: extrathoracic (ET), tracheobronchial (BB), bronchioles (bb), and pulmonary (PU). Systemic dosimetry estimates were made for nonreactive (Category 3) gases. Results suggest similar ET dosimetry for children and adults for all types of inhaled materials. BB dosimetry was also similar across age groups except that the dosimetry of ultrafine particles in this region was twofold greater in 3-mo-old children than in adults. In contrast, the bb region generally showed higher dosimetry of particles and gases in adults than in children. Particle dose in the PU region was two- to fourfold higher in 3-mo-old children, with the greatest child/adult difference occurring for submicron size particles. Particulate dosimetry estimates with the default RfC methodology were below those found with the ICRP model for both adults and children for submicrometer sized particles. There were no cases in which reactive gas dosimetry was substantially greater in the respiratory regions of 3-mo-old children. Estimates of systemic dose of Category 3 gases were greater in 3-mo-old children than in adults, especially for liver dose of metabolite for rapidly metabolized gases. These analyses support the approach of assuming twofold greater inhalation dose in children than adults, although there are cases in which this differential can be greater and others where it can be less.

ADVANCES IN RISK ASSESSMENT AND COMMUNICATION

Risk analysis continues to evolve. There is increasing depth and breadth to each component of the four-step risk-assessment paradigm of hazard identification, dose-response analysis, exposure assessment, and risk characterization. Basic conceptual approaches to understanding how people perceive risk are being tested against a growing body of empirical observations, many involving stakeholders. Emerging ideas such as the precautionary principle have provided challenges that have led to a rethinking of the role of risk assessment in environmental health. Newer problems, such as intergenerational issues posed by long-lasting radiation pollution, environmental justice, and the assessment and communication of risks related to terrorism, have spurred innovative approaches to risk analysis.

PCB contamination from polysulphide sealants in residential areas-exposure and risk assessment

From the late 1950s to the early 1970s elastic polysulphide sealants were used in outdoor seams between concrete blocks in prefabricated buildings. The sealants contained 5-30% polychlorinated biphenyls (PCBs). Due to the weathering of sealants in general and the replacement of seams with new PCB-free materials in the 1990s, PCBs have drifted to the soil adjacent to buildings. The objectives of this study were to survey PCB contamination in the surroundings of former PCB-containing buildings and to evaluate the risks to human health. Samples from soil, and also from blood serum of residents, were collected to obtain data for exposure assessment. The health risk assessment was based on deterministic and probabilistic calculations for cancer and non-cancer risks. Soil ingestion and dermal contact were considered the main routes of exposure and children the most important exposed group. The mean total PCB concentration was 6.83 mg/kg within 2m of the buildings and 0.52 mg/kg within 3-10 m from of the buildings. The deterministic risk assessment with conservative parameters resulted in lifetime cancer risk estimates on the order of 10(-6)-10(-7). The lifetime average daily dose (LADD) for PCBs was less than 10% of the reference dose (RfD) 0.02 microg/kg day, which is based on immunosupression in monkeys. The LADD corresponding to the total site attributable exposure was less than 10% of the estimated average dietary PCB intake in Finland. Children can, however, in worst cases be exposed to daily doses near the level of the RfD. Low cost measures are recommended to reduce possible exposure of children.

Age-related differences in susceptibility to carcinogenesis. II. Approaches for application and uncertainty analyses for individual genetically acting carcinogens

In an earlier report we developed a quantitative likelihood-based analysis of the differences in sensitivity of rodents to mutagenic carcinogens across three life stages (fetal, birth to weaning, and weaning to 60 days) relative to exposures in adult life. Here we draw implications for assessing human risks for full lifetime exposures, taking into account three types of uncertainties in making projections from the rodent data: uncertainty in the central estimates of the life-stage-specific sensitivity factors estimated earlier, uncertainty from chemical-to-chemical differences in life-stage-specific sensitivities for carcinogenesis, and uncertainty in the mapping of rodent life stages to human ages/exposure periods. Among the uncertainties analyzed, the mapping of rodent life stages to human ages/exposure periods is most important quantitatively (a range of several-fold in estimates of the duration of the human equivalent of the highest sensitivity "birth to weaning" period in rodents). The combined effects of these uncertainties are estimated with Monte Carlo analyses. Overall, the estimated population arithmetic mean risk from lifetime exposures at a constant milligrams per kilogram body weight level to a generic mutagenic carcinogen is about 2.8-fold larger than expected from adult-only exposure with 5-95% confidence limits of 1.5- to 6-fold. The mean estimates for the 0- to 2-year and 2- to 15-year periods are about 35-55% larger than the 10- and 3-fold sensitivity factor adjustments recently proposed by the U.S. Environmental Protection Agency. The present results are based on data for only nine chemicals, including five mutagens. Risk inferences will be altered as data become available for other chemicals.

Physiologically-based pharmacokinetic and toxicokinetic models in cancer risk assessment

Physiologically-based pharmacokinetic (PBPK) and toxicokinetic models are increasingly being used for the conduct of high dose to low dose and interspecies extrapolations required in cancer risk assessment. These models, by simulating tissue dose of toxic chemicals, help address the uncertainty associated with the default approaches for interspecies and high dose to low dose extrapolations. The applicability of PBPK models in cancer risk assessment has been demonstrated with a number of chemicals (e.g., acrylonitrile, 2-butoxyethanol, chloroform, 1,4-dioxane, methyl chloroform, methylene chloride, styrene, trichloroethylene, tetrachloroethylene, vinyl chloride, vinyl acetate). Recent advances in PBPK modeling facilitate the consideration of population distribution of parameter values, age-dependent changes in physiology and metabolism, multi-route exposures as well as multichemical interactions for application in cancer risk assessment. Whereas the average values for various input parameters have been used to evaluate the age-dependency of tissue dose, the Markov Chain Monte Carlo technique can be applied to address variability and uncertainty in parameter estimates, thus facilitating a more accurate estimation of cancer risk in the population. The PBPK models also uniquely facilitate the simulation of tissue dose, and thereby cancer risks, associated with multi-route and multichemical exposure situations. Overall, the recent advances reviewed in this article point to the continued enhancement of the scientific basis and applicability of PBPK models in cancer risk assessment.

Children as a sensitive subpopulation for the risk assessment process

For cancer risk assessment purposes, it is necessary to consider how to incorporate sensitive subpopulations into the process to ensure that they are appropriately protected. Children represent one such potentially sensitive subpopulation that is of quite considerable magnitude. The data needs include sensitivity to the induction of childhood cancers compared to adult cancers and relative sensitivity of early-life exposures for the formation of tumors in adults. These needs as far as human data are concerned are best met for ionizing radiations, for which it has been shown in the atomic bomb survivors that early-life exposures are more effective at inducing cancers later in life. The risk assessment approach for ionizing radiations, however, is based on tumor data itself for total population exposures so that there is no requirement to consider specifically the impact of early-life exposures. In the case of environmental chemicals, the majority of the tumor data used for risk assessments are from rodent bioassays. There is a paucity of data that allow for a comparison of the response to early-life exposures compared to that for adult-only exposures. This presents a fairly difficult challenge to the identification of a general sensitivity factor or a chemical-specific sensitivity factor for early-life exposures. The U.S. Environmental Protection Agency (EPA) has not, until recently, incorporated a general adjustment for early-life exposure to carcinogens into its risk assessment guidelines. The Agency has relied on the fact that, in the absence of specific data to the contrary, the linear extrapolation for rodent tumor data provided appropriate protection. When specific data are available, then an adjustment can be calculated. In its most recent draft guidelines, however, a general adjustment has been proposed for mutagenic chemicals. A 10-fold risk adjustment is recommended for the first 2 years of life, a 3-fold adjustment for years 3-15, and no adjustment for exposures after age 15. For chemicals that do not have a mutagenic mode of action, no adjustment is recommended because the data for deriving such an adjustment are simply not available. Clearly, this is an interim position that is dependent on more pertinent data being collected. A significant component of this is to conduct cancer bioassays that include early-life exposures.

Evaluation of the Potential Impact of Age- and Gender-Specific Pharmacokinetic Differences on Tissue Dosimetry 2Current address: Novartis Pharmaceuticals, East Hanover, NJ 07936

The physiological and biochemical processes that determine the tissue concentration time courses (pharmacokinetics) of xenobiotics vary, in some cases significantly, with age and gender. While it is known that age- and gender-specific differences have the potential to affect tissue concentrations and, hence, individual risk, the relative importance of the contributing processes and the quantitative impact of these differences for various life stages are not well characterized. The objective of this study was to identify age- and gender-specific differences in physiological and biochemical processes that affect tissue dosimetry and integrate them into a predictive physiologically based pharmacokinetic (PBPK) life-stage model. The life-stage model was exercised for several environmental chemicals with a variety of physicochemical, biochemical, and mode-of-action properties. In general, predictions of average pharmacokinetic dose metrics for a chemical across life stages were within a factor of two, although larger transient variations were predicted, particularly during the neonatal period. The most important age-dependent pharmacokinetic factor appears to be the potential for decreased clearance of a toxic chemical in the perinatal period due to the immaturity of many metabolic enzyme systems, although this same factor may also reduce the production of a reactive metabolite. Given the potential for age-dependent pharmacodynamic factors during early life, there may be chemicals and health outcomes for which decreased clearance over a relatively brief period could have a substantial impact on risk.

Physiologically based pharmacokinetic (PBPK) modeling of caffeine and theophylline in neonates and adults: implications for assessing children's risks from environmental agents

Children's risks can differ from those in adults for numerous reasons, one being differences in the pharmacokinetic handling of chemicals. Immature metabolism and a variety of other factors in neonates can affect chemical disposition and clearance. These factors can be incorporated into physiologically based pharmacokinetic (PBPK) models that simulate the fate of environmental toxicants in both children and adults. PBPK models are most informative when supported by empirical data, but typically pediatric pharmacokinetic data for toxicants are not available. In contrast, pharmacokinetic data in children are readily available for therapeutic drugs. The current analysis utilizes data for caffeine and theophylline, closely related xanthines that are both cytochrome P-450 (CYP) 1A2 substrates, in developing PBPK models for neonates and adults. Model development involved scale-up of in vitro metabolic parameters to whole liver and adjusting metabolic function for the ontological pattern of CYP1A2 and other CYPs. Model runs were able to simulate the large differences in half-life and clearance between neonates and adults. Further, the models were able to reproduce the faster metabolic clearance of theophylline relative to caffeine in neonates. This differential between xanthines was found to be due primarily to an extra metabolic pathway available to theophylline, back-methylation to caffeine, that is not available to caffeine itself. This pathway is not observed in adults exemplifying the importance of secondary or novel routes of metabolism in the immature liver. Greater CYP2E1 metabolism of theophylline relative to caffeine in neonates also occurs. Neonatal PBPK models developed for these drugs may be adapted to other CYP1A2 substrates (e.g., arylamine toxicants). A stepwise approach for modeling environmental toxicants in children is proposed.