A distributional approach to characterizing low-dose cancer risk

Since cancer risk at very low doses cannot be directly measured in humans or animals, mathematical extrapolation models and scientific judgment are required. This article demonstrates a probabilistic approach to carcinogen risk assessment that employs probability trees, subjective probabilities, and standard bootstrapping procedures. The probabilistic approach is applied to the carcinogenic risk of formaldehyde in environmental and occupational settings. Sensitivity analyses illustrate conditional estimates of risk for each path in the probability tree. Fundamental mechanistic uncertainties are characterized. A strength of the analysis is the explicit treatment of alternative beliefs about pharmacokinetics and pharmacodynamics. The resulting probability distributions on cancer risk are compared with the point estimates reported by federal agencies. Limitations of the approach are discussed as well as future research directions.

Reasoning, decision making and rationality

It is argued that reasoning in the real world supports decision making and is aimed at the achievement of goals. A distinction is developed between two notions of rationality: rationality which is reasoning in such a way as to achieve one's goals--within cognitive constraints--and rationality which is reasoning by a process of logic. This dichotomy is related to the philosophical distinction between practical and theoretical reasoning. It is argued that logicality (rationality) does not provide a good basis for rationality and some psychological research on deductive reasoning is re-examined in this light. First, we review belief bias effects in syllogistic reasoning, and argue that the phenomena do not support the interpretations of irrationality that are often placed upon them. Second, we review and discuss recent studies of deontic reasoning in the Wason selection task, which demonstrate the decision making, and rational nature of reasoning in realistic contexts. The final section of the paper examines contemporary decision theory and shows how it fails, in comparable manner to formal logic, to provide an adequate model for assessing the rationality of human reasoning and decision making.

Workers' breath as a source of perchloroethylene (Perc) in the home

Wallace et al. (1991a) measured elevated levels of perchloroethylene (Perc) in the homes of six dry cleaning workers who used Perc in the workplace. This paper considers the hypothesis that dry cleaning workers introduce Perc into their homes by exhaling Perc accumulated in their bodies during the work day. We simulated worker exposure considering current occupational guidelines, uptake and body burden using a physiologically-based pharmacokinetic (PBPK) model for Perc, and home concentrations using a simple model for a "typical" home. These results suggest that workers' body burdens of Perc may be sufficient to explain elevated home concentrations. The implications of these results, for design of exposure monitoring studies, are discussed, along with some other policy implications.

The value of animal test information in environmental control decisions

Value of information (VOI) analytic techniques are used to evaluate the benefit of performing animal bioassays to provide information about the cancer potency of specific chemical compounds. These tools allow the identification of the conditions in which the cost of reducing uncertainty about potency, by performing a subchronic or chronic bioassay, is justified by the benefit of having improved information for making control decisions. The decision analytic results are readily scaled to apply to a range of human contact rates (exposures) and a variety of control strategies. The sensitivity of results to uncertainty about animal to human extrapolation and the design of the bioassay is explored. An evaluation of the possible gains in general understanding about the mechanisms of carcinogenicity resulting from chronic bioassays is beyond the scope of this approach.

The mental model theory of conditional reasoning: critical appraisal and revision

Johnson-Laird and Byrne (1991) present a theory of conditional inference based upon the manipulation of mental models. In the present paper, the theory is critically examined with regard to its ability to account for psychological data, principally with respect to the rate at which people draw the four basic inferences of modus ponens (MP), denial of the antecedent (DA), affirmation of the consequent (AC) and modus tollens (MT). It is argued first that the theory is unclear in its definition and in particular with regard to predictions of problem difficulty. Clarification and specification of principles are consequently provided here. Next, it is argued that there are a number of phenomena in the conditional reasoning literature for which the theory cannot account in its present form. Specifically, (a) the relatively frequency of DA and AC inferences on affirmative conditional is not as predicted by the theory, (b) differences occur between inferences on if then and only if rules beyond the capacity of the theory to explain and (c) there is no account of the "negative conclusion bias" observed when negated components are introduced into the rules. A number of revisions to the mental model theory of conditional reasoning are proposed in order to account for these findings.

An empirical model for estimating the collection efficiency of dust-mist respirators

This paper compares experimental measurements of filter performance with predictions of aerosol collection by dust-mist respirators using the single fibre efficiency model and an empirical model developed by the authors. The former, a theoretically-based semi-empirical model, indicates that mechanical phenomena such as impaction, interception and diffusion are responsible for only a fraction of the collection efficiency of dust-mist respirators, i.e. 30-70% depending on particle size. The difference between these relatively low predicted efficiencies and experimentally observed values, which were greater than 95%, can be attributed to electrostatic forces. Because it was not possible to determine filter fibre charge the single fibre efficiency model could not be used to predict electrostatic collection. An empirical model developed from experimental data with monodisperse latex aerosols was used to predict silica and asbestos collection. The empirical model adequately described silica collection, but predicted much higher asbestos penetration than was observed experimentally.

The source of belief bias effects in syllogistic reasoning

In studies of the belief bias effect in syllogistic reasoning, an interaction between logical validity and the believability of the conclusion has been found; in essence, logic has a larger effect on unbelievable than on believable conclusions. Two main explanations have been proposed for this finding. The selective scrutiny account claims that people focus on the conclusion and only engage in logical processing if this is found to be unbelievable; while the misinterpreted necessity account claims that subjects misunderstand what is meant by logical necessity and respond on the basis of believability when indeterminate syllogisms are presented. Experiments 1 and 2 compared the predictions of these two theories by examining whether the interaction would disappear if only determinate syllogisms were used. It did, thus providing strong support for the misinterpreted necessity explanation. However, the results are also consistent with a version of the mental models theory, and so Experiment 3 was carried out to compare these two explanations. The mental models theory received strong support, as it did also in the follow-up Experiments 4 and 5. It is concluded that people try to construct a mental model of the premises but, if there is a believable conclusion consistent with the first model they produce, then they fail to construct alternative models.

Alternative management of choledocholithiasis

This paper is presented to clarify the role of ERCP in conjunction with laparoscopic cholecystectomy. As the field of surgery moves more toward less invasive procedures, our management schemata may also need revision. With the popularity of laparoscopic cholecystectomy came a large increase in the number of patients treated with cholelithiasis, and therefore an increase in patients with choledocholithiasis. The authors believe that treatment options exist to avoid open common bile duct procedure in most cases.

An early review of 800 laparoscopic cholecystectomies at a university-affiliated community teaching hospital

Laparoscopic cholecystectomy (LC) was first performed at Georgia Baptist Medical Center (GBMC) in December 1989, subsequently becoming the treatment of choice for most patients with symptomatic gallbladder disease. Early in the authors' series, all patients evaluated for cholecystitis were treated laparoscopically, unless the third party refused reimbursement or the attending surgeon was not trained in LC. Indications for LC were no different than for standard open cholecystectomy (OC). Eight hundred patients from December 1989 to March 1991 had an attempted LC at GBMC. The procedure was completed in 782 patients (97.7%) and required conversion to OC in 18 patients, (2.3%) primarily because of technical difficulties such as dense adhesions or gangrenous changes. No patient sustained a trocar injury to the intra-abdominal viscera, bile ducts injury, or major vascular injury. Overall morbidity was 3.1 per cent and mortality 0.13 per cent. Selective cholangiography (SIOC) was used in 14 per cent. Endoscopic retrograde cholangiopancreatography (ERCP), choledochoscopy, and Fogarty catheter techniques were used for common bile duct stone management. Average hospitalization was 0.89 days, with 85 per cent discharged in less than 24 hours. Average operative time was 86 minutes (range: 25 to 353). Patients returned to full activities at home in 8.4 days. Savings on hospital charges to patients averaged $1,100 for inpatient LC and $2,500 for outpatient LC when compared to 1989 costs for OC. Laparoscopic cholecystectomy is the current surgical procedure of choice for most patients with cholecystitis and can be done at least as safely as standard open cholecystectomy. The morbidity appears to be significantly less with LC, but longer follow-up is needed to confirm these preliminary findings.(ABSTRACT TRUNCATED AT 250 WORDS)

Multipractice analysis of laparoscopic cholecystectomy in 1,983 patients

We analyzed the results of laparoscopic cholecystectomy in 1,983 patients from a variety of practice settings in order to evaluate a large, cross-sectional experience for this new procedure. Twenty general surgeons from 9 clinics in 4 states examined the records and outcome of their laparoscopic cholecystectomy patients through March 1991. In 88 patients (4.5%), the operation was converted to an open procedure, usually because of marked inflammation and unclear anatomy. A total of 644 cases were performed with laser dissection and 1,339 with cautery, and the results of these 2 methods were similar. There were 41 complications. Reoperation for repair was necessary in 18 patients, including 5 with common duct injuries, and, to date, the outcome has been good in each patient. Seventy-six patients (3.8%) have had recognized common duct stones; these were removed preoperatively by endoscopic sphincterotomy (ERS) in 20 patients, during cholecystectomy in 46 patients, and postoperatively by ERS in 4 patients. In six patients, common duct stones became apparent 1 to 4 months after cholecystectomy. We conclude that trained general surgeons can perform laparoscopic cholecystectomy safely with risks comparable to those for conventional open cholecystectomy.

Effects of age and biotin status on postnatal development of plasma biotinidase activity in rats

Biotinidase activity was measured in plasmas of 1-, 7-, 14-, and 21-day-old rats from control dams and dams that had been fed a biotin-depleting diet from Day 15 of gestation. Biotinidase activity increased significantly in the plasma of rats from control and depleted mothers until Postnatal Day 14, after which there was a small but significant decline at Day 21. Differences between the mean activities of the two groups of pups on each sampling day were not significant and there were no significant differences in activity levels attributable to sex. Plasma albumin concentrations increased from birth until Day 21, and plasma biotinidase activity and albumin concentration were significantly correlated (r = +/- 0.43). We suggested that these two proteins may be controlled by a common mechanism in the early postnatal period, and that biotin deficiency does not affect the development of biotinidase activity. Because biotin-depleted neonatal pups show developmental changes in biotinidase activity similar to those of human newborns, and they can be produced reliably by depleting dams from Day 15 of gestation, they may be useful models for studying the developmental abnormalities associated with human biotinidase deficiency.

Biliary infusion therapy in the inspissated bile syndrome of cystic fibrosis

A 3.5-month-old white boy was born with meconium ileus, peritonitis, and jejunal atresia from cystic fibrosis. He subsequently developed unrelenting and severe extrahepatic biliary obstruction as demonstrated by liver biopsy showing periportal inflammation, cholestasis, and fibrosis. Surgical exploration confirmed the diagnosis of extrahepatic biliary obstruction by severely inspissated bile. A cholecystostomy tube was left in place. The cholestasis remained unresponsive to conservative medical therapy. The obstruction was relieved by hydrostatic infusion of 2% N-acetylcysteine into the biliary tree over a 6-day period. The child also received concurrently four i.v. injections of synthetic cholecystokinin. This therapeutic modality was thought to be both safe and effective.

Structure and function of X-Pro dipeptide repeats in the TonB proteins of Salmonella typhimurium and Escherichia coli

The TonB protein is required for several outer membrane transport processes in bacteria. A short 33-residue peptide segment of TonB has been studied by 1H and 13C nuclear magnetic resonance spectroscopy. The sequence of this peptide segment contains multiple Glu-Pro and Lys-Pro dipeptide repeats that maintain rigid, elongated structures and flank a short connecting segment that adopts a beta-strand configuration. This TonB peptide is shown to interact specifically with the FhuA protein, the outer membrane receptor for ferrichrome-iron, providing the first direct evidence that the TonB protein interacts with outer membrane receptors. Interaction with the FhuA protein involves the extended structural element containing positively charged Lys-Pro repeats, and suggests a functional role for this segment of the TonB protein. As TonB is anchored in the cytoplasmic membrane the protein must, uniquely, span the periplasm. These data, together with studies described in the accompanying paper, suggest a model by which TonB serves to transduce conformational information over extended distances, from the cytoplasmic membrane to the outer membrane.

TonB protein of Salmonella typhimurium. A model for signal transduction between membranes

The tonB gene product is required for several outer membrane transport processes in bacteria. The tonB gene from Salmonella typhimurium was sequenced and found to be similar to that of Escherichia coli. The TonB protein is highly proline-rich and includes an unusual segment consisting of multiple X-Pro dipeptide repeats. A synthetic peptide corresponding to this segment has been used to raise anti-TonB antibodies. TonB was shown to be associated with the cytoplasmic membrane, apparently anchored via a single hydrophobic N-terminal segment. Protease accessibility studies, and the use of a series of TonB-beta-lactamase fusions, showed that the rest of the TonB protein is periplasmic. Unusually, export of TonB is not accompanied by cleavage of the N-terminal signal peptide. In the accompanying paper, we show that TonB interacts directly with the outer membrane FhuA (TonA) receptor. Thus, TonB must span the periplasm, providing a link between the cytoplasmic membrane and receptors in the outer membrane. On the basis of these data, and those published by other laboratories, we propose a model whereby TonB serves as a "mechanical" linkage that, by transmitting protein conformational changes from the cytoplasmic membrane across the periplasm, acts as a means of coupling energy to outer membrane transport processes. Such a mechanism has general implications for signal transduction within and between proteins.

Collection of silica and asbestos aerosols by respirators at steady and cyclic flow

Air purifying dust/mist respirators are presently tested using a silica aerosol under conditions of steady flow. In experiments, the predictive validity of such tests was evaluated by testing respirators using silica and asbestos aerosols under conditions of both steady and cyclic flow. Silica penetration at steady flow was reasonably predictive of silica penetration under cyclic flow. However, asbestos penetration under cyclic flow was not predicted well by penetration of either silica or asbestos at steady flow. Furthermore, the potential for exhalation valve failure under cyclic flow was identified. Current NIOSH protocols for evaluating respirator performance should be reconsidered in light of these findings.

Degradation of thiophene-2-carboxylate, furan-2-carboxylate, pyrrole-2-carboxylate and other thiophene derivatives by the bacterium Vibrio YC1

A bacterium of the genus Vibrio, capable of degrading thiophene derivatives, was isolated from enrichment cultures inoculated with oil-contaminated estuarine mud. Of the thiophene derivatives tested, only thiophene-2-carboxylate (T2C), thiophene-2-acetate and thiophene-2-carbonyl chloride supported growth, but a total of seven were significantly oxidised by resting cells. Furan-2-carboxylate (F2C) and pyrrole-2-carboxylate (P2C) also supported growth and were oxidised. The heteroatoms of T2C and P2C were released into the media as sulphate and ammonia and served as sole sources of sulphur and nitrogen for growth. Mutants defective in T2C utilisation (Thc-) were isolated, many being also defective in F2C and P2C utilisation.

Collection efficiency of respirator filters challenged with monodisperse latex aerosols

Collection efficiency was evaluated for 10 respirator manufacturers' electrostatically-charged dust/mist filters challenged with eight sizes of latex spheres in a range representative of the silica aerosol used in National Institute for Occupational Safety and Health (NIOSH) certification tests. Minimum efficiency occurred at or below the smallest size of 0.102 microns. Appreciable differences were found in the performances of filters distributed by the 10 manufacturers. Filters produced by 1 manufacturer exhibited significantly lower collection efficiency than other filters tested; those produced by 2 manufacturers performed somewhat better than the others tested. Statistical evaluation of Weibull-transformed data using analyses of covariance and Tukey's significant difference test allowed classification of the 10 manufacturers into three performance categories.

The distribution of Student's t-statistic for small samples from lognormal exposure distributions

To assess compliance with industrial hygiene exposure criteria (e.g., TLVs), it may be necessary to perform statistical tests of hypotheses based on relatively small samples. For pollutants with long biological half-lives, the parameter most relevant for determining the risk faced by workers is the long-term arithmetic average concentration of the pollutant. In industrial environments it is common for pollutant concentrations to be approximately lognormal. Unfortunately, when based on small samples from lognormal distributions, the ordinary Student's t-statistic has some undesirable characteristics which are not recognized widely by practicing industrial hygienists. The difficulties in using the ordinary Student's t-statistic to evaluate the average exposure have been demonstrated. The properties of alternative test statistics have been explored. Some general observations on the implications of these findings have been made.

Occult blood in stools and necrotizing enterocolitis. Is there a relationship?

A prospective study was undertaken to determine the incidence of occult hematochezia in enterally fed low-birth-weight neonates (less than 1800 g at birth). The relationship of this occult hematochezia to the development of necrotizing enterocolitis (NEC) was then analyzed. Daily stool specimens from 95 neonates were tested for occult blood during the first six weeks of life. Fifty-four (58%) of the 95 neonates had one or more blood-positive stools. Six neonates (6.3%) developed NEC. Necrotizing enterocolitis occurred in only two of the 54 neonates with one or more blood-positive stools vs four of the 41 neonates with blood-negative stools only. Presence of occult blood in the stools did not correlate with development of NEC.

Biosynthesis of type I procollagen. Characterization of the distribution of chain sizes and extent of hydroxylation of polysome-associated pro-alpha-chains

[3H]Proline-labeled nascent procollagen chains were isolated from chick tendon polysome preparations as peptidyl-tRNA complexes by ion exchange chromatography. Proline hydroxylation of the nascent chains was at least 40% complete, based on radioactive hydroxyproline/proline ratios. These data provide the first direct evidence that hydroxylation of procollagen proline residues does occur on nascent chains. The electrophoretic profiles of [3H]proline-labeled nascent chains and of unlabeled nascent chains visualized by Western blotting with 35S-labeled monoclonal antibodies to the alpha 1(I) N-propeptide or the C-propeptides indicate that there are pauses in the translation of procollagen alpha-chains in the intact cells. Approximately 25% of the radioactivity associated with [3H]proline-labeled polysomes was in fully elongated but underhydroxylated (relative to secreted procollagen) pro-alpha-chains. The association of these completely elongated but only partially modified procollagen chains with the polysome complex may facilitate the carboxyl-terminal interactions which lead to triple helix formation.

Sequence-imposed structural constraints in the TonB protein of E. coli

The solution conformation of a 33-residue peptide segment, derived from the TonB protein which is implicated in bacterial membrane transport processes, has been investigated using high-resolution proton magnetic resonance techniques. This proline-rich peptide possesses sequence-imposed sections of elongated secondary structure that must be retained in the native protein configuration. These structural constraints provide elements of stiffness that imply a purely structural role for TonB and are relevant to the subcellular location and biological role of the protein. On the basis of these data we suggest that this protein spans the periplasmic space, linking the inner and outer membrane components of TonB-dependent transport systems.

The influence of diagnostic radiography on the incidence of breast cancer and leukemia

Little accurate information is available on the influence of diagnostic radiography on the incidence of cancer in the general population, largely because of a lack of knowledge of the numbers and the age distributions of patients undergoing radiographic examinations. We used such information from a closed population in Maine that was followed for a year, along with data from the literature on the absorbed dose from typical examinations and a mathematical model linking dose to the incidence rate of cancer, to estimate the numbers of cases of radiologically induced cancer of the bone marrow and breast. Our data indicate that about 1 percent of all cases of leukemia and less than 1 percent of all cases of breast cancer result from diagnostic radiography. In addition, such tumors generally occur late in life; the incidence of radiation-induced leukemia was highest at 69 years, and the incidence of radiation-induced breast cancer was highest at 76 years. Thus, diagnostic radiography has only a small influence on the occurrence of leukemia and breast cancer.