Invisible air pollution in the country may be just as toxic as the smog of the city

It's easy to see air pollution when it settles as smog over a city. Yet even in rural areas, where the air looks clearer, researchers have found unhealthy particles floating in the atmosphere.

That sweet country air may not be as invigorating as we once thought.

Fine particulate matter less than 2.5 micrometers in diameter (PM2.5) is thought to cause the most harm to human health, as these pollutants are small enough to seep deep into our lungs, damaging the cells and tissues that reside there.

As guidance, the World Health Organization has therefore set a safety threshold for ambient PM2.5 levels, and yet this line in the sand overlooks the nuances of intrinsically toxic chemicals.

Emerging research suggests the mass of fine particulate matter we breathe could be less important for human health than its chemical makeup.

That's because some lighter particles are more likely to produce reactive oxygen species, which can have toxic effects on human health.

When researchers in the United States compared three urban areas to one rural area in the midwest, they found similar levels of oxidative potential at all four sites. That was true even though the rural site had a relatively lower mass of PM2.5.

While agricultural activities only contributed 12 percent of the rural site's PM2.5 mass, they accounted for more than 60 percent of the region's cellular oxidative potential.

The oxidative potential of most urban sites, on the other hand, was less than 54 percent.

"Overall, our study indicates that the sources contributing substantially to PM2.5 mass are not necessarily equally important in terms of their health effects," the authors write in a new paper.

Instead, the researchers argue our health metrics for air pollution should be based more on the toxic potential of fine particles than their actual mass.

The study is based on weekly samples of PM2.5, which were retrieved in the summer and fall of 2018 and the winter and spring of 2019 from Chicago, Indianapolis, and St Louis, as well as a rural location in Illinois.

Analyzing the composition, mass, and oxidative potential of these samples, the team found a poor correlation between the mass and toxicity of fine particulate matter.

Lighter chemicals in rural areas were much more likely to produce unhealthy byproducts.

Floating traces of iron and organic carbon, for instance, were strongly correlated with cellular oxidative potential throughout the year. Other industrial chemicals, like lead, aluminum, copper, and manganese, tended to increase during winter and fall.

The strong seasonality of these results suggests many of the potentially toxic chemicals being breathed in rural Illinois are due to agricultural activities, like the application of fertilizers and herbicides.

Phosphate fertilizers that are sprayed on crops, for instance, contain heavy-metal pollutants, like lead and chromium, that can easily infiltrate the air and our lungs. Copper fungicides are also sprayed in a similar way.

Despite being located 12 kilometers (7 miles) from a coal-fired power plant, coal combustion and biomass burning accounted for more than 80 percent of rural Illinois' PM2.5 mass.

If the authors had simply measured PM2.5, it would look like coal was the most dangerous factor for human health. But that may not be true. Instead, agricultural sources, which are lighter in mass, appeared to be twice as toxic as burnt biomass.

"[D]espite a minor contribution to PM2.5 mass, health risks of the agricultural activities cannot be ignored," the authors conclude.

Nor can we afford to ignore other lighter forms of fine particulate matter.

For instance, a study conducted in Beijing in 2019 found vehicle emissions only contributed 10 percent of the region's PM2.5 mass. Yet they accounted for more than half of all the toxic potentials measured in the air.

Clearly, the way that we are measuring air pollution is flawed, and is not capturing the full extent of harm.

Measuring the oxidative potentials of individual chemicals, however, is much trickier than simply weighing the mass of all ambient pollutants. The authors of the current study hope their new methodology can make testing for toxic air easier for environmental regulators and policymakers going forward.

The study was published in the Journal of Hazardous Materials. 

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