How Clean is Clean Enough?

Good Job America

Air in the US has been getting cleaner for several decades, thanks to the Clean Air Act (CAA). The image below shows how emissions have decreased over this time period. This is good, because it means that we have fewer illnesses and deaths due to poor air quality. What scientists do not know, is whether there is some threshold where air is clean enough that there are no negative impacts. Scientists have to keep studying this issue. As the air gets cleaner, scientists keep studying how health is associated with air quality to see if people are still getting sick from the air.

Image from shows how air pollution has decreased sharply even as the population and economy have grown, showing the success of regulations. The PM2.5 and ozone studied here are part of the “six common pollutants”

Keep Going

Some people are more likely to get sick from poor air quality than others. One group of people that is particularly susceptible is the elderly population. Researchers analyzed all deaths in the Medicare population from 2000 to 2012 and looked at the air quality on that day in the location where that person lived. They used both computer models and measurements of small particulate matter (PM2.5) and ozone. By combining several different forms of air quality data, they were able to get a better representation of pollution levels across the whole US than other studies. They have information on pollution levels in 33% more zip codes than they would have if they had just looked at locations with a monitoring station. This is particularly useful for researching low levels of air pollution, because more measurements tend to be taken of dirtier air, since we already know that area is problematic for health.

The current National Ambient Air Quality Standards (NAAQS) for PM2.5 requires less than 25 micrograms per cubic meter (ug/m3) of air averaged over the day and less than 70 parts per billion (ppb) of ozone measured as 8 hour maximum. Researchers analyzed days where air pollution concentrations were less than 25 ug/m3 of PM2.5 and 60 ppb of ozone, which is well below the current standard. They then estimated curves for how exposure to each pollutant relates to mortality using statistical methods. These curves expand upon the previous knowledge about how human health is impacted by relatively clean air.

What does it mean?

The results show that air quality that meets current standards is still associated with increased mortality in a susceptible population. Even low levels of air pollution are associated with an increase in mortality. The analysis does not show a threshold below which cleaner air stops providing health benefits. The EPA is required to review the standards every 5 years. The results of this study suggest that there is evidence the standards should be strengthened.

The association between air pollution and mortality is well known, but this study enhances our knowledge. It uses a very large sample, more than 22 million deaths, to provide robust statistical results. The evaluation of the relationship at low levels of air pollution also enhances an area where less was known about the relationship. The study is specific to the Medicare population, primarily individuals older than 65 years, but more than two-thirds of deaths in the US occur in people over 65 years of age, so much of the impact of air quality on mortality is captured.


Source Article: Association of Short-Term Exposure to Air Pollution with Mortality in Older Adults DOI:10.1001/jama.2017.17923

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Kristen Brown

Kristen Brown

I am a postdoctoral researcher at the EPA where I specialize in evaluating environmental impacts of our energy system. I have a PhD in Environmental Engineering from CU Boulder where I also received a master’s in Mechanical Engineering, and I have a BA in Physics from Cal Berkeley. Outside of work, I’m an amateur boxer and have two spoiled dogs. You can follow me on twitter at @Kris10BrownPhD and find out about my research at

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