Particulate matter (PM) is extremely diverse, and comes from many sources. It ranges in sizes from a few nanometers to a few microns, both of which are far smaller than anything seen by the naked eye.Source: EPA

Particulate Matter Air Pollution: Here’s Why The Facts Matter

RICK PELTIER, ASSOCIATE PROFESSOR, 

Up in the air. And everywhere.

Air pollution is a complicated mixture that includes both invisible gases and solid particles or liquid droplets that float in the air. These pollutants are everywhere—in your home and workplace, in your neighborhood park, near busy (and not-so busy) roadways. And they can make us very sick.

This week, scientific experts are deliberating the science on how these particles and droplets affect health. Combined, these are known as particulate matter (PM) and they are one of the more complicated pollutants that the EPA is required to regulate. It takes a team of knowledgeable scientists to connect our exposures to PM to a wide range of public health problems that we see in society. And protecting public health is important for all Americans.

PM is extremely diverse, and comes from many sources. It ranges in sizes from a few nanometers to a few microns, both of which are far smaller than anything seen by the naked eye. The sources of these particles are equally as diverse, and come from things like car and truck exhaust, emissions from industrial smokestacks, and can even form in the atmosphere from precursors that combine to create new particles by chemical reactions. They can change in shape and chemical composition over time, and they travel in any direction that the wind takes them. Thousands of them are inhaled into our lungs with every breath we take, which occur 12-16 times per minute, 24 hours per day.

A complicated association with health

The EPA regulates PM in terms of its weight per volume of air. It doesn’t make any difference if that PM is extremely toxic, or has no health impact at all—everything is based on weight. In atmospheric research, we call this a ‘bulk’ measurement, because it treats all forms of PM the same. It is a little like monitoring calories and different kinds of foods—if we try to keep to 2000 calories a day (a bulk measurement), we can fill our diet with healthy fruits and vegetables, or just a few unhealthy donuts. In reality, we fill our diets with a combination of both.

In a similar way, what we breathe is a mixture of PM that is a combination of quite toxic particles, as well as those that might be less harmful. Because we don’t always know which particles are harmful, we rely on independent scientific consensus to figure out what concentration of PM (a bulk measurement) most likely protects health with an adequate margin of safety. And scientists know that Americans have no choice but to breathe whatever types of particles happen to be around us.

Scientists can’t be absolutely certain about how many extra deaths in the US are caused by particulate matter, but it is in the range of 30,000-80,000 per year. To put this into context, these are more deaths per year from PM than from car accidents and gun deaths (combined), and on par with similar numbers of breast cancer or diabetes deaths. But attributing deaths directly to particulate matter exposure is not straightforward, because PM is actually a trigger for other life-threatening diseases such as cardiopulmonary disease, cancer, and asthma, among others. Making this accounting—linking PM exposures to varied health endpoints—is challenging, but not impossible, and it’s why having independent scientists who can volunteer their expertise is so critical to the federal policy review process.

We often have only limited information on PM on its chemical makeup and its toxicity. PM concentrations also vary enormously in different locations, and even by different times of day or season. Because of this tremendous complexity, understanding how PM affects health is especially difficult and requires sophisticated expertise that can tease out the relationships between exposure and human health.

It would be best if we could simply test how specific types of PM affect people under highly controlled conditions, but this approach has serious ethical concerns and is usually prohibitively expensive. As a result, we have to study what happens using observations in the real-world environment. This means following different populations of people: some who live and work in extreme pollution, others who live or work in moderate pollution, and yet others who live and work in relatively clean environments.  Often, these types of studies involve hundreds or many thousands of people, and sometimes, the conclusions are inconsistent. This is because both people and particles are diverse, and some particles are hazardous to certain populations, and others are not. This is why we need robust and independent scientists, who can identify different relationships of exposure, to guide sound air quality policy.

Good science benefits us all

As a professor, I have plenty of things that fill my work week—I teach and manage a large lab, mentor students, analyze data and write research papers, and sit on countless university committees. And the scientists who are meeting have equally full plates. But all scientists are on a relentless and objective search for truth, and we volunteer our time to tease out this truth, despite our many other professional obligations.

It is outrageous that the EPA is so dismissive of objective science, and I’m grateful for the volunteers electing to meet this week in their search for truth. For me, the search for scientific truth is worth the additional workload, because there is a broader impact to this quest; I know that policy crafted on good science benefits us all.