Tightening the Ambient Air Quality Standard for 'Soot': They’re at It Again!

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The New York Times reported that the US Environmental Protection Agency (EPA) will reconsider reducing the National Ambient Air Quality Standard (NAAQS) from its current 12 μg/m3 as an annual average. What’s new, and what’s a tired retread? 

What’s new?

  • Reducing the NAAQS to 9 μg/m3.
  • Claiming that reduction to that level would save 12,150 additional lives each year.
  • Associating COVID mortality with PM2.5.

What is the same-old-same-old?

  • Confusing PM2.5, a mixture of many types of particles classified only by size, with soot or unburned carbon. 
  • Conflating association with causation.
  • Recognizing uncertainties in the underlying health studies, including lack of demonstrated effects.
  • Ignoring the limitations of EPA’s ambient air quality monitoring system.
  • Neglecting indoor air quality.
  • Assuming that large stationary sources are the primary emitters of PM2.5.

Let’s address the issues one at a time.

  • The New York State Department of Health describes PM2.5 as “tiny particles in the air that reduce visibility and cause the air to appear hazy.” Wikipedia describes soot as “impure carbon particles resulting from … incomplete combustion.” Soot particles with aerodynamic diameters less than 2.5 microns would be classified as PM2.5; these classifications are not interchangeable.
  • PM2.5 has never been shown to cause an increased incidence of chronic diseases, including cardiopulmonary diseases. Such proof would have to show that the incidence increased in proportion to long-term exposure and decreased as that exposure was reduced after accounting for confounding. Statistical association under steady conditions cannot be considered causal. Causation must also consider latency and cumulative exposures, as with tobacco or occupational exposures. Long-term ambient air quality studies have not met these challenges. 
  • Long-term mortality studies follow either of two approaches. In cohort studies, individuals are recruited and then followed until enough have died to allow robust statistical analysis. Cohort studies must consider contextual factors such as climate, density, access to green space. None of the cohort studies that EPA has used to support regulations have considered the totality of contextural issues. In population studies, published mortality rates are analyzed for population statistics such as age, race, income, and socioeconomic status (SES). Population studies cannot account for critical individual characteristics, notably smoking and body mass index.   
  • Air pollution health studies require relevant measures of exposure, preferably for individuals. They are seldom available in numbers sufficient for mortality studies. EPA air quality data is primarily intended to assess compliance with the NAAQS rather than to estimate health-related exposures. As a result, data on the constituents of PM2.5, including the carbon compounds relevant to “soot,” are sparse. Moreover, only regulated pollutants are routinely monitored; the lack of ambient data on a suspected harmful agent such as “soot” precludes adequate investigation. 
  • We spend up to 90% of our time indoors, but those environments have never been considered in establishing NAAQS levels. It’s especially important for PM2.5 because of the many indoor sources, notably smoking, which can increase indoor levels by up to 20 μg/m3. Part of the rationale for reducing the PM2.5 NAAQS is the failure of epidemiology studies to find health effect thresholds based on outdoor air quality. Until indoor exposures are considered, the relevant exposure levels will continue to be underestimated, even with the NAAQS set to zero. 
  • There is a pervasive thread throughout the NY Times article implying that large stationary industrial facilities are the primary despoilers of the environment. While that assumption had some validity in previous decades, this is no longer the case. Ambient PM2.5 is widely distributed and cannot be linked to a specific source; traffic and indoor, domestic activities contribute much more to personal exposures than fixed sources.
  • The health endpoints of most concern, cancer, and cardiorespiratory conditions, have long latency periods such that today’s monitoring data may not be relevant for decades to come. 

For the new issues before the EPA:

  • The EPA’s reports on existing ambient air quality are very limited, comprising only 104 of our 3116 counties, and are limited to observations every six days for PM2.5. The 9 μg/mlevel was exceeded in only nine counties, 8% of those surveyed, 0.2% of all of our nation’s counties. 
  • Lives cannot be “saved” but may be extended for a variety of reasons. Life extensions linked to ambient air pollution reductions have been estimated at a matter of weeks, but such extensions have never been directly observed. 
  • Relationships between COVID-19 mortality and PM2.5 have been reported as representing long-term exposures that may have decreased the ability to resist viral infections. However, those studies have not considered the exposures of previous decades that would have been required to produce such outcomes, nor have they considered changes in the spatial distributions of COVID-19 infections that have occurred during the course of the pandemic.[1]

Protection of public health through cleaner air can only be achieved by understanding all of the exposures, timing, and mechanisms of plausible effects. Such understanding must be a prerequisite for cost-effective, science-based regulation.    

[1] COVID-19 and the Environment, Review and Analysis. Environments DOI: 10.3390/environments805004

Source: NY Times EPA to Review Rules on Soot Linked to Deaths, Which Trump Declined to Tighten