A tightening of the annual PM2.5 standard to 8 µg/m3 would remove the differences in exposure between different ethnic groups. Photo: © oneinchpunch / Shutterstock.com

New US standard for fine particles to be expected

The US Environmental Protection Agency (US EPA) proposes to tighten national air quality standards for fine particle matter (PM2.5) for the first time since 2012.

The US EPA proposes to lower the annual average standard for PM2.5 to between 8-11 µg/m3, from the current 12 µg/m3. The US EPA is tasked to set two types of National Ambient Air Quality Standards (NAAQS). The standard that they want to update is a so called “primary standard,”, which is a health-based standard that should consider the health of sensitive or at-risk groups, with an adequate margin of safety. There are also “secondary standards ” that aims to protect public welfare in general. The US EPA states in their review that they do not want to change their secondary standards. Neither the primary 24-hour PM2.5 standard, currently set at the level of 35 µg/m3.

As part of its review, the US EPA, enlists the help of a committee of independent scientists known as The Clean Air Scientific Advisory Committee (CASAC). In a document to the US EPA, CASAC express their agreement on the inadequacy of the current annual PM2.5 standard (12 µg/m3), “all CASAC members agree that the current level of the annual standard is not sufficiently protective of public health and should be lowered”.  However, there was some disagreement about how much it should be lowered. The majority of CASAC members “judge[d] that an annual average in the range of 8–10 μg/m3” was most appropriate. Worth noting is that they also recommended that the level of the current primary 24-hour PM2.5 should be revised to within the range of 25 to 30 mg/m3, while a minority recommended retaining the current standard. Here the US EPA deviates in its proposal from CASAC's recommendations.

As a basis for its proposal, the US EPA has produced two reports: the Integrated Science Assessment (ISA) for Particulate Matter published in 2019 and the Supplement to the original report published three years later. It is solid work based on thousands of scientific articles. Both have since been evaluated and commented on by CASAC.

The first report identified causal relationships, such as that between long- and short-term exposures to PM2.5 and cardiovascular effects, respiratory effects, nervous system effects and cancer, as well as adverse health effects at levels below the current annual PM2.5 standard level. The Supplement provides more in-depth studies of the identified health effect categories. CASAC, notes that there is a progression going from the previous Integrated Science Assessment published in 2009 to the more recent reports indicating continued strengthening of the causal health endpoints relationship with PM2.5. This reflects the growing body of literature that show strong associations with health effects, even though concentrations of PM2.5 in the air have been decreasing over time.

In the latest reports, PM2.5 was also found to unequally affect minority populations and populations with low socioeconomic status. The studies continue to provide evidence indicating that associations with PM2.5 are independent of other air pollutants and factors that also could influence the association such as lifestyle factors. It further concluded that epidemiological studies conducted to date do not identify a population-level threshold below which it can be concluded with confidence that PM2.5-related effects do not occur. Human exposure studies also support the causality. Accountability studies, where we can study the effect of a policy or closure of industrial facilities and corresponding reduction in the number of cases, also play an important part in assessment. Here, too, analyses find significant associations with reduced exposure to PM2.5, even below the current standards, and decreased health effects.

CASAC has provided consensus advice on the need to revise the level of the primary annual PM2.5 standard to a level below the current standard to provide additional protection from PM2.5-related health effects. To enhance protection of air quality, especially in overburdened and vulnerable communities where there are environmental justice concerns due to disproportionate air pollution risks, the EPA is proposing to modify the PM2.5 monitoring network design criteria to include an environmental justice factor. The agency is also proposing changes to the Air Quality Index (AQI) to reflect the proposed changes to the primary annual PM2.5 standard and reflect recent science on PM2.5 and health.

The EPA judgments are today often considered as requisite (i.e., neither more nor less stringent than necessary) to protect public health with an adequate margin of safety. According to a ruling in the Supreme Court in 2001, known as Whitman v. American Trucking Associations, Inc., the EPA cannot consider costs when setting or revising NAAQS. Similar judgments have ruled that neither economic nor technological feasibility should be considered when setting air quality standards. The absence of any provision requiring consideration of these factors was no accident; it was the result of a deliberate decision by Congress to subordinate such concerns to the achievement of health goals. In the preparatory work for the Clean Air Act, it had been determined that 1) the health of people is more important than the question of whether the early achievement of ambient air quality standards protective of health is technically feasible; and 2) the growth of the pollution load in many areas, even µg/with the application of available technology, would still be deleterious to public health. In the Lead Industry vs EPA (1980) case this was further investigated. “Subclinical” effects, that is, symptoms that are only detectable by physical examination or laboratory test, should not be considered as adverse health effects that are clearly harmful unless this has been proven, but an adequate margin of safety was allowed.

In the decision of proposing new standards, CASAC chose a conservative approach. Instead of looking at the lowest levels of detecting health effects, they focused on the study-reported mean of the PM2.5 air quality distribution limited to US multi-city or multi-state studies. They noted that there is no specific point in the air quality distribution of any epidemiologic study that represents a “bright line” at, and above which effects have been observed and below which effects have not been observed. Naturally, the bulk of the health events (high data density in the middle portions of the distributions) in each study have been observed, generally at or around the mean concentration. The mean exposure in the chosen US studies were between 8-17 µg/m3. They did report that they identified effects at lower levels (6 and 9 µg/m3), in US studies that provided data also on effects at the lowest quarter of exposure. One study reported effects even lower (5 µg/m3). Looking at Canadian studies with lower means of exposure, effects were identified between 6-10 µg/m3.  The proposed limit values, however, reflect the mean of US studies. The requirement to provide an adequate margin of safety was intended to address uncertainties associated with inconclusive scientific and technical information and to provide a reasonable degree of protection against hazards that research has not yet identified. The margin of safety has not been clearly assessed and the EPA welcomes comments on including effects seen at lower levels or international studies that have been able to study effects in low exposure areas.

When this evaluation was done a health risk assessment was conducted to look at what impact different policy options between 8–12 µg/m3 would have on the population. Although the EPA cannot consider costs in setting or revising NAAQS, they nevertheless analyse the benefits and costs of implementing the standards for the purpose of informing the public. Compared to the current annual standards, air quality adjusted to meet the alternative level of 8 µg/m3 had a four times  larger reduction in mortality than the alternative level of 11 µg/m3.) Under the hypothetical air quality scenarios, disparities exist between different ethnicities with regards to both PM2.5 exposures and PM2.5-attributable mortality risk rates under the current PM NAAQS. When considering the lowest alternative annual standard evaluated – an alternative annual standard of 8 µg/m3 – disparities in exposure are virtually eliminated. The public is now welcome with comments to the proposal of primary annual and 24-hour PM2.5 standards.

Ebba Malmqvist

Proposed Decision for the Reconsideration of the National Ambient Air Quality Standards for Particulate Matter (PM) Link: https://www.epa.gov/pm-pollution/proposed-decision-reconsideration-natio...


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