One plus one does not equal two - it is more. Researchers have developed a method to analyse the toxic effects of mixed air pollutants. The method reveals greater health damage from poor air quality than analyses of individual substances suggest.
Text: Annika Lund, first published in Medicinsk Vetenskap nr 2 2025
Fine particles are considered particularly harmful to health. One reason is that many small particles together form a very large surface area - and that is where other toxic substances attach. One example is PAHs (polycyclic aromatic hydrocarbons). a group of substances released by traffic, industrial processes, or wood burning. Some are cancer-causing.
PAHs are the focus of toxicologist Kristian Dreij , who leads a research group at the Institute of Environmental Medicine at Karolinska Institutet.
"Current practice is to measure and study the substances found in air pollution one at a time. Our view is that we need to look at the entire complex mixture present in the air, because that is what we breathe. We believe this is necessary to truly understand the health risks of air pollution," says Kristian Dreij.
Mixtures are the worst
His research group has shown that signals indicating DNA damage in cells give a good indication of the potential of individual substances to cause cancer. When they used the same method to measure the potential of air samples containing a complex mix of pollutants, they found a greater cancer-causing potential than that of the individual substances in the air sample.
"There are thousands of substances in an air sample. For the vast majority, we lack knowledge of their effects. Some of these substances may cause DNA damage, others may affect defence systems that make cells more vulnerable, or have other effects. That is our overarching theory as to why the mixtures are more harmful," says Kristian Dreij.
Air samples from the Amazon were carcinogenic
His group has analysed air samples from five cities in Japan, Sweden and Brazil. Air samples from areas where large amounts of biomass are burned turned out to be more potentially carcinogenic. These samples came from regions in the Amazon where forest burning contributes to air pollution.
The researchers want to move forward in several ways. One path is to try to understand why smoke from forest fires appears particularly toxic. This understanding could provide new insights into lung cancer development and help validate the testing method.
In another study, still in the planning stage, the researchers aim to collect air samples from Stockholm over the course of a year. After analysis, the results would be compared with lung cancer incidence in Stockholm.
In the long term, the research group hopes to develop a relatively simple test to assess air samples. This could help better identify where air quality interventions are most needed.
"But we also have a broader idea. There is a risk that the threshold values we usetoday, based on individual substances, do not give an accurate picture of health risks. In the long run, we believe air samples should be assessed in a better way, taking the mixture of pollutants into account. If so, air quality limits may need to be revised," says Kristian Dreij.
