What's wrong with the National Ambient Air Quality Standard (NAAQS) for fine particulate matter (PM(2.5))?

Systematic review of the association between long-term exposure to fine particulate matter and mortality

We used a transparent systematic review framework based on best practices for evaluating study quality and integrating evidence to conduct a review of the available epidemiology studies evaluating associations between long-term exposure to ambient concentrations of PM_2.5 and mortality (all-cause and non-accidental) conducted in North America. We found that while there is some consistency across studies for reporting positive associations, these associations are weak and several important methodological issues have led to uncertainties with regard to the evidence from these studies, including potential confounding by measured and unmeasured factors, exposue measurement error, and model misspecification. These uncertainties provide a plausible, alternative explanation to causality for the weakly positive findings across studies. Using a causality framework that incorporates best practices for making causal determinations, we concluded that the evidence for a causal relationship between long-term exposure to ambient PM_2.5 concentrations and mortality from these studies is inadequate.

On the techniques and standards of particulate matter sampling

Air pollution and its limits are regulated by the environmental protection agency of an individual country according to their National Ambient Air Quality Standards (NAAQS). Particulate matter (e.g., TSP, PM₁₀, and PM_2.5) is one of the important criteria pollutants of NAAQS. Their measurement methods are specified in NAAQS, and detailed technical descriptions are given in standards. This review focuses on the sampling and analysis techniques and methods in the context of PM samplers' design mentioned in countries specific PM measuring standards (e.g., EPA Part 50, CEN 12341, IS 5182(23), etc.) and their comparison wherever is necessary. It discusses, different designs of PM samplers mentioned in standards and its important components, e.g., size fractionators cutoff efficiency, PM sampler head design, flow measurement, and calibration, and also addresses the important issues that are the limitation of present standards. Our review reveals that most of the country-specific standards show common practice in measuring PM_2.5 using WINS impactor and VSCC cyclone as mentioned in EPA Part 50, except European Union (EU) standards, which has different design and parameters. For PM₁₀ measurement, sampler design is different in EU and Indian standards than that of U.S. EPA and other countries' standards, which is discussed in length here. All standards lack in pointing some inherent problems like change in D₅₀ cutoff of size fractionator of sampler under a high particle mass loading condition, which is common in countries like China and India. Other important issues where most of the standards lack include PM head design and specification, a key component of PM sampler on which the mass measurement results are largely dependent.Implications: The review paper discusses the air quality standards compliances of different countries and their comparisons. It focuses on the sampling and analysis techniques in context of PM samplers' design mentioned in countries specific PM measuring standards, and also addresses the important issues that are not mentioned in standards. Therefore, the discussions and findings of the review may be very useful while revising the existing air quality standards of different countries and to fill the research gap in this domain. Further, we have discussed several technical issues described in standards related to PM sampling which may be very helpful for PM sampler designing or modification in current designs as per the prevailing ambient conditions of a country.

A Systematic Review of Air Quality Sensors, Guidelines, and Measurement Studies for Indoor Air Quality Management

The existence of indoor air pollutants—such as ozone, carbon monoxide, carbon dioxide, sulfur dioxide, nitrogen dioxide, particulate matter, and total volatile organic compounds—is evidently a critical issue for human health. Over the past decade, various international agencies have continually refined and updated the quantitative air quality guidelines and standards in order to meet the requirements for indoor air quality management. This paper first provides a systematic review of the existing air quality guidelines and standards implemented by different agencies, which include the Ambient Air Quality Standards (NAAQS); the World Health Organization (WHO); the Occupational Safety and Health Administration (OSHA); the American Conference of Governmental Industrial Hygienists (ACGIH); the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE); the National Institute for Occupational Safety and Health (NIOSH); and the California ambient air quality standards (CAAQS). It then adds to this by providing a state-of-art review of the existing low-cost air quality sensor (LCAQS) technologies, and analyzes the corresponding specifications, such as the typical detection range, measurement tolerance or repeatability, data resolution, response time, supply current, and market price. Finally, it briefly reviews a sequence (array) of field measurement studies, which focuses on the technical measurement characteristics and their data analysis approaches.

Investigating measurements of fine particle (PM2.5 ) emissions from the cooking of meals and mitigating exposure using a cooker hood

There is growing awareness that indoor exposure to particulate matter with diameter ≤ 2.5 μm (PM_2.5 ) is associated with an increased risk of adverse health effects. Cooking is a key indoor source of PM_2.5 and an activity conducted daily in most homes. Population scale models can predict occupant exposures to PM_2.5 , but these predictions are sensitive to the emission rates used. Reported emission rates are highly variable and are typically for the cooking of single ingredients and not full meals. Accordingly, there is a need to assess PM_2.5 emissions from the cooking of complete meals. Mean PM_2.5 emission rates and source strengths were measured for four complete meals. Temporal PM_2.5 concentrations and particle size distributions were recorded using an optical particle counter (OPC), and gravimetric sampling was used to determine calibration factors. Mean emission rates and source strengths varied between 0.54-3.7 mg/min and 15-68 mg, respectively, with 95% confidence. Using a cooker hood (apparent capture efficiency > 90%) and frying in non-stick pans were found to significantly reduce emissions. OPC calibration factors varied between 1.5 and 5.0 showing that a single value cannot be used for all meals and that gravimetric sampling is necessary when measuring PM_2.5 concentrations in kitchens.

Is PM More Toxic Than the Sum of Its Parts? Risk-Assessment Toxicity Factorsvs.PM-Mortality “Effect Functions”

Epidemiology studies of populations living in areas with good air quality report correlations between levels of ambient particulate matter (PM) and mortality rates. These associations occur at low PM concentrations that are below current air quality standards. Can such concentrations cause mortality, given the toxicity of PM chemical constituents? We examined chemical-specific, dose-response data typically used in U.S. EPA human health risk assessments. These assessments rely on established, no-effect thresholds for noncancer health endpoints. We found that chemicals identified as constituents of ambient PM are present at concentrations considerably below the regulatory thresholds used in risk assessment (i.e., below the RfCs and RfDs that identify levels for which no adverse health effects are anticipated). From the perspective of risk assessment, exposure to the concentrations of chemicals in ambient PM (e.g., sulfate, nitrate, and elemental carbon) cannot be expected to cause death. Hence, the health effects attributed to ambient PM in "regulatory impact analyses" appear to be at odds with what would be predicted from a standard U.S. EPA health-risk assessment for PM chemicals. Four possible resolutions of this paradox are that (1) the mixtures of chemicals present in ambient PM are vastly more toxic than the sum of individual components, (2) small portions of the general population are vastly more sensitive to certain ambient PM chemicals than reflected in U.S. EPA toxicity factors, (3) the toxicity of ambient PM is unrelated to its chemical constituents, or (4) PM mass concentration is not the causal factor in the reported associations. The associations may arise because ambient PM concentrations (1) are a surrogate for unmeasured copollutants (e.g., HAPs), (2) covary with confounding factors that cannot be fully controlled (e.g., weather, demographics), or (3) covary with unmeasured (e.g., societal, behavioral, or stress) factors.

The Big Ban on Bituminous Coal Sales Revisited: Serious Epidemics and Pronounced Trends Feign Excess Mortality Previously Attributed to Heavy Black-Smoke Exposure

The effect of banning bituminous coal sales on the black-smoke concentration and the mortality rates in Dublin, Ireland, has been analyzed recently. Based on the application of standard epidemiological procedures, the authors concluded that, as a result of the ban, the total nontrauma death rate was reduced strongly (-8.0% unadjusted, -5.7% adjusted). The purpose of this study was to reanalyze the original data with the aim of clarifying the three most important aspects of the study, (a) the effect of epidemics, (b) the trends in mortality rates due to advances in public health care, and (c) the correlation between mortality rates and black-smoke concentrations. Particular attention has been devoted to a detailed evaluation of the time dependence of mortality rates, stratified by season. Death rates were found to be strongly enhanced during three severe pre-ban winter-spring epidemics. The cardiovascular mortality rates exhibited a continuous decrease over the whole study period, in general accordance with trends in the rest of Ireland. These two effects can fully account for the previously identified apparent correlation between reduced mortality and the very pronounced ban-related lowering of the black-smoke concentration. The third important finding was that in nonepidemic pre-ban seasons even large changes in the concentration of black smoke had no detectable effect on mortality rates. The reanalysis suggests that epidemiological studies exploring the effect of ambient particulate matter on mortality require improved tools allowing proper adjustment for epidemics and trends. Aspects of harvesting and more recent results derived from a distributed lag model covering the effects of black smoke and temperature are also discussed.

Exposures of healthy and asthmatic volunteers to concentrated ambient ultrafine particles in Los Angeles

Adult volunteers (17 healthy, 14 asthmatic) were exposed in a controlled environmental chamber to concentrated ultrafine particles (UFP) collected in a Los Angeles suburb with substantial motor vehicle pollution. Exposures lasted 2 h with intermittent exercise. Inhaled particle counts (mean 145,000/cm(3), range 39,000-312,000) were typically 7-8 times higher than ambient levels. Mass concentrations (mean 100 microg/m(3), range 13-277) were not highly correlated with counts. Volunteers were evaluated for lung function, symptoms, exhaled nitric oxide (eNO), Holter electrocardiography, and inflammatory markers in peripheral blood and induced sputum. Relative to control (filtered air) studies, UFP exposures were associated with a 0.5% mean fall in arterial O(2) saturation estimated by pulse oximetry (p < .01), a 2% mean fall in forced expired volume in 1 sec (FEV(1)) the morning after exposure (p < .05), and a transient slight decrease in low-frequency (sympathetic) power in Holter recordings during quiet rest (p < .05). Healthy and asthmatic subjects were not significantly different across most endpoints. Thus, this initial experimental study of human volunteers exposed to concentrated Los Angeles area ambient UFP showed some acute deleterious cardiopulmonary responses, which, although generally small and equivocal as in previous studies of larger sized concentrated ambient particles, might help to explain reported adverse health effects associated with urban particulate pollution.

Short-term effects of particulate matter: an inflammatory mechanism?

"Would you tell me please, which way I ought to go from here," asked Alice. "That depends a good deal on where you want to go to," said the cat. (Lewis Carroll, Alice's Adventures in Wonderland) A large number of epidemiological studies show positive correlations between increasing levels of particulate matter (PM) in urban air and short-term morbidity and mortality for diverse acute cardiopulmonary diseases. Brought about by PM increments, inflammation is thought to exacerbate preexisting inflammatory diseases. Experimental evidence suggests a hierarchical oxidative stress model, in which a weakened antioxidant defense, as observed in disease or induced by inhaled particles, increases the PM ability to cause lung inflammation, accounting for exacerbations that occur in asthmatics and in patients with chronic obstructive lung disease. The role of PM-induced inflammation leading to acute cardiovascular events such as arrhythmia, heart failure, and myocardial infarction is more speculative. There is neither clear-cut evidence in humans that inhaled PM could get as far as blood circulation nor that proinflammatory mediators are significantly released from inflamed lung tissues, nor that blood coagulability is critically altered. As a whole, data in humans indicate that short-term inflammatory responses to PM are not always detected; they are usually mild and loosely correlated with functional changes. Among these studies, the diversity of PM characteristics, dose metrics, and endpoints hampers a clear discerning of inflammatory mechanism(s). Thus, the question arises as to whether inflammation represents the mechanism of acute cardiopulmonary PM toxicities in susceptible individuals, or rather an event that may coexist with other relevant mechanism(s). This review article discusses the evidence in humans linking short-term PM increments to inflammation and to exacerbations of cardiopulmonary diseases. Although there is a large amount of data available, there still remains a gulf between the number of epidemiological and panel studies and that of controlled exposures. Research on controlled exposure needs expanding, so that the results of time-series and panel studies will be better understood and short-term standards for human exposure may be more confidently allocated.

In search of the most relevant parameter for quantifying lung inflammatory response to nanoparticle exposure: particle number, surface area, or what?

BACKGROUND

Little is known about the mechanisms involved in lung inflammation caused by the inhalation or instillation of nanoparticles. Current research focuses on identifying the particle parameter that can serve as a proper dose metric.

OBJECTIVES

The purpose of this study was to review published dose-response data on acute lung inflammation in rats and mice after instillation of titanium dioxide particles or six types of carbon nanoparticles. I explored four types of dose metrics: the number of particles, the joint length--that is, the product of particle number and mean size--and the surface area defined in two different ways.

FINDINGS

With the exception of the particle size-based surface area, all other parameters worked quite well as dose metrics, with the particle number tending to work best. The apparent mystery of three equally useful dose metrics could be explained. Linear dose-response relationships were identified at sufficiently low doses, with no evidence of a dose threshold below which nanoparticle instillation ceased to cause inflammation. In appropriately reduced form, the results for three different sets of response parameters agreed quite well, indicating internal consistency of the data. The reduced data revealed particle-specific differences in surface toxicity of the carbon nanoparticles, by up to a factor of four, with diesel soot being at the low end.

CONCLUSIONS

The analysis suggests that the physical characterization of nanoparticles and the methods to determine surface toxicity have to be improved significantly before the appropriate dose metric for lung inflammation can be identified safely. There is also a need for refinements in quantifying response to exposure.

Respiratory responses to exposures with fine particulates and nitrogen dioxide in the elderly with and without COPD

Elderly people, with and without chronic obstructive pulmonary disease (COPD), may be susceptible to particulate matter (PM) air pollution. However, the respiratory impacts of inhaled PM combined with copollutant(s) in controlled exposure studies are unclear and warrant investigation since exposures to PMgas mixtures constitute realistic scenarios. Thus, we exposed 6 healthy subjects and 18 volunteers with COPD (mean age 71 yr) on separate days to (a) filtered air (FA); (b) 0.4 ppm NO2; (c) concentrated ambient particles (CAP), predominantly in the fine (PM2.5) size range, at concentrations near 200 microg/m3; and (d) CAP and NO2 together. Each 2-h exposure included exercise for 15 min every half hour. Most respiratory responses, including symptoms, spirometry, and total and differential counts of induced sputum cells, showed no statistically significant responses attributable to separate or combined effects of CAP and NO2. However, maximal mid-expiratory flow and arterial O2 saturation (measured by pulse oximetry) showed small but statistically significant decrements associated with CAP, greater in healthy than COPD subjects. CAP exposure was also associated with decreased percentages of columnar epithelial cells in sputum. The results suggest that the respiratory effect of the PMNO2 mixture may be primarily PM driven since coexposure to NO2 did not significantly enhance the responses. In conclusion, older adults exposed to urban fine particles may experience acute small-airways dysfunction with impaired gas exchange. Healthy subjects appear more susceptible, suggesting that the respiratory effect may be related to efficient penetration and deposition of inhaled toxic particles in distal small airways. More clinical investigation of the elderly population is warranted.

Genotoxicity of size-fractionated samples of urban particulate matter

Urban particulate matter (UPM) includes particles of size smaller than 10 microm (PM10), which may impact on human respiratory and cardiovascular health. It has been reported previously that PM10 can induce DNA damage. We have collected size-fractionated PM10 at the roadside and measured the induction of DNA damage by different-sized UPM using the alkaline Comet assay and the plasmid strand-break assay. We found that foil disks were more suitable for collecting UPM than quartz fiber filters, as the UPM could be easily extracted from the foil disks and accurately weighed. Using the Comet assay, all size fractions induced DNA damage in A549 lung epithelial cells, with the finer fractions (D50% = 0.65 microm and lower) inducing the most damage. In the plasmid strand-break assay, in which DNA damage is induced by free-radical species generated in solution, the most damage was also induced by the finer fractions, although the finest fraction (D50% < 0.43 microm) did not induce as much damage as D50% = 0.65 and 0.43 microm. When an organic extract of a standard UPM sample was compared to the whole particles and the washed particles in the Comet assay, it was found that around 75% of the damage induced by the whole UPM could be induced by the organic extract. These results show that finer particulates have the greatest ability to induce DNA damage in lung epithelial cells and naked DNA, and that both organic and inorganic components of the UPM contribute to its genotoxic effects.

Ambient Gas Concentrations and Personal Particulate Matter Exposures

BACKGROUND

Data from a previous study conducted in Baltimore, MD, showed that ambient fine particulate matter less than 2.5 mum in diameter (PM2.5) concentrations were strongly correlated with corresponding personal PM2.5 exposures, whereas ambient O3, NO2, and SO2 concentrations were weakly correlated with their personal exposures to these gases. In contrast, many of the ambient gas concentrations were reasonable surrogates of personal PM2.5 exposures.

METHODS

Personal multipollutant exposures and corresponding ambient air pollution concentrations were measured for 43 subjects living in Boston, MA. The cohort consisted of 20 healthy senior citizens and 23 schoolchildren. Simultaneous 24-hour integrated PM2.5, O3, NO2, and SO2 personal exposures and ambient concentrations were measured. All PM2.5 samples were also analyzed for SO4 (sulfate). We analyzed personal exposure and ambient concentration data using correlation and mixed model regression analyses to examine relationships among (1) ambient PM2.5 concentrations and corresponding ambient gas concentrations; (2) ambient PM2.5 and gas concentrations and their respective personal exposures; (3) ambient gas concentrations and corresponding personal PM2.5 exposures; and (4) personal PM2.5 exposures and corresponding personal gas exposures.

RESULTS

We found substantial correlations between ambient PM2.5 concentrations and corresponding personal exposures over the course of time. Additionally, our results support the earlier finding that summertime gaseous pollutant concentrations may be better surrogates of personal PM2.5 exposures (especially personal exposures to PM2.5 of ambient origin) than they are surrogates of personal exposures to the gases themselves.

CONCLUSIONS

Particle health effects studies that include both ambient PM2.5 and gaseous concentrations as independent variables must be analyzed carefully and interpreted cautiously, since both parameters may be serving as surrogates for PM2.5 exposures.

Dosimetry and toxicology of ultrafine particles

While epidemiological studies indicate an association between adverse health effects and ambient ultrafine particle concentrations in susceptible individuals, toxicological studies aim to identify mechanisms which are causal for the gradual transition from the physiological status towards patho-physiological disease. Impressive progress has been made in recent years when objectives changed from classical tests like lung function, etc. to endpoints comprising of particle induced oxidative stress, cell signaling and activation, release of mediators initiating inflammatory processes not only in the respiratory tract but also in the cardio-vascular system. Particularly, the large surface area of ultrafine particles provides a unique interface for catalytic reactions of surface-located agents with biological targets like proteins, cells, etc. However, toxicological studies are hampered by a number of imminent complications when simulating long-term exposure of humans in urban environments with inherited and/or acquired susceptibility (e.g., acute exposure studies at high concentrations either in human subjects or animal models). Yet, based on a conservative estimate results available begin to show an adverse health risk for susceptible individuals and support the epidemiological evidence.

Particulate matter in ambient air and mortality: toxicologic perspectives

U.S. regulations that set standards for acceptable concentrations of respirable particulate matter (PM) in outdoor air, particularly total fine particulate matter (PM(2.5)), are based largely on the belief that current concentrations cause death and illness, and that reducing these concentrations will save lives. Because the mortality risk estimates from important observational epidemiologic studies are extremely weak, derived from studies unable to control for relevant confounding causes, and inconsistent by location, toxicologic and clinical information is necessary to judge the likelihood and degree to which such findings are causal. Toxicologic data on typical forms of pollution-derived PM strongly suggest that current ambient concentrations in the U.S. are too small to cause significant disease or death. We review here the results of inhalation studies using concentrated ambient particles, diesel engine exhaust particulate matter, and sulfate and nitrate salts, and find no evidence that moderate concentrations are lethal. The expectation that lives will be saved by reducing ambient PM(2.5) in the U.S. is not supported by the weight of scientific evidence, although other bases for regulating PM may be justifiable.