Pollution linked to road traffic could have a greater impact than previously thought on mortality among people with cancer. A study published in Nature Sustainability reached this conclusion after analyzing more than 9.2 million deaths in eight countries.
The researchers focused on PM2.5, fine particles capable of penetrating deep into the lungs. Some can even enter the bloodstream. The data studied covered different periods between 2000 and 2019, depending on the country.
The main originality of the work lies in distinguishing all PM2.5 particles from those attributed to road traffic. To do so, the team relied on atmospheric models and several emissions inventories.
The contrast was clear. A rise of 10 micrograms per cubic meter of PM2.5 over two days was associated with a 0.77% increase in mortality. For traffic-related particles, this increase reached 3.87%.
This difference is all the more surprising because traffic particles accounted for only 14.72% of the estimated total concentrations. Yet the model attributed 73.55% of PM2.5-associated cancer deaths to them. Across the study, this would represent approximately 112,000 deaths.
The differences also varied according to the type of cancer. For lung cancer, the increase reached 4.10% for an additional 10 micrograms of traffic-related PM2.5. It rose to 5.57% for colorectal cancers and 6.27% for prostate cancer.
The composition of these particles could play a role. Traffic produces more than just exhaust emissions. Brake, tire and road wear also release particles, as does the resuspension of dust. It is therefore the very presence of traffic that must be taken into account, rather than just the energy used by vehicles. Two identical PM2.5 concentrations can thus conceal very different mixtures.
To reduce bias, the researchers used a method called “case-crossover.” Each deceased person is compared with themselves on nearby days serving as references. This approach limits the influence of stable individual characteristics, while the models also incorporate several weather-related factors.
The study does not, however, directly measure each person’s exposure. The share of pollution attributed to traffic is based on modeling, which introduces a margin of uncertainty. Nevertheless, the authors point to several plausible mechanisms, including inflammatory, cardiovascular or respiratory effects that could rapidly worsen the condition of vulnerable people.