Air pollution blights the lives of untold millions in urban areas worldwide. Long-term exposure to even relatively low levels of polluted air can cause or worsen a wide variety of health problems. So combating air pollution is a must.

Turns out, though, that doing so isn’t as simple and straightforward a matter as we might think.

Concentrations of atmospheric particulate matter known as PM2.5 (airborne particles with a diameter of less than 2.5 micrometers, or a mere fraction of the diameter of a human hair) are a main indicator of the degree of pollution present in the air. By reducing those levels of PM2.5 we reduce air pollution.

The trouble is, argue scientists in a new study, that reducing the level of PM2.5 in polluted air could have an unintended consequence. In higher concentrations PM2.5 suppresses the formation of ultrafine particles because these larger particles absorb smaller ones.

These ultrafine particles are generated, among other things, by diesel exhaust, which is abundant in many towns and cities. If concentrations of PM2.5 are reduced, then smaller particles can proliferate, which can lead to dire consequences for our health.

“Ultrafine particles are generally made from combustion processes in which growth of the particle is from molecules upwards and so these particles can be extremely small,” a medical journal explains. Increases in pollution from these miniscule particles, it adds, “have been found to be associated with a range of adverse health effects and these are very well documented.”

If we reduce concentrations of PM2.5 and ignore the buildup of ultrafine particles, we risk making matters even worse, the scientists behind the new study warn. This does not mean that reducing high concentrations of PM2.5 should be less of a priority. Rather, we will need to tackle both types of airborne pollutants simultaneously, the researchers stress, if we are to have really clean air in our cities.

“I[G]reat care is needed to avoid inadvertently worsening the situation by reducing the mass of airborne particulate matter, only to increase… the numbers and the toxicity of the ultrafine [particles] as a result,” a professor of environmental science at Lancester University stresses.

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