Traffic pollution may help to reduce HDL cholesterol

A wealth of studies have suggested a link between exposure to air pollution and a greater risk of cardiovascular disease. But according to an epidemiological study, adults who were exposed to traffic-related air pollution had lower levels of high-density lipoprotein (HDL) cholesterol that could increase their cardiovascular disease risk. 

For the new research, lead author Griffith Bell, Ph.D., of the University of Washington School of Public Health in Seattle, and colleagues studied 6,654 adults and found people exposed to higher levels of fine and ultrafine particles in traffic pollution tended to have lower levels of HDL cholesterol in their blood.
Published in the journal Arteriosclerosis, Thrombosis, and Vascular Biology, the new study suggests that adults who reside in areas with greater air pollution – particularly traffic-related air pollution – may have lower levels of high-density lipoprotein (HDL) cholesterol.

Researchers focused on so-called PM 2.5, a mixture of solid particles and liquid droplets smaller than 2.5 micrometers in diameter that can include dust, dirt, soot and smoke. They also looked at what’s known as black carbon, a component of particulate pollution that’s formed by burning various kinds of fuels.

Study participants were 62 years old on average, and half of them were current or former smokers. About 16 percent of the participants took cholesterol-lowering drugs and roughly 45 percent had high blood pressure. None had cardiovascular disease at the start of the study period.

Over one year, people exposed to more black carbon had lower levels of HDL cholesterol than participants with little or no exposure to black carbon. The difference was small, but statistically meaningful.

“However, this was a fairly small effect – it wasn’t a dramatic lowering of HDL – so I don’t think this is a huge cause for alarm beyond what we already know about the dangers of air pollution,” said lead study author Griffith Bell.

The study wasn’t a controlled experiment designed to prove how traffic fumes directly influence cholesterol or the risk of heart disease.

Although both men and women experienced reductions in HDL cholesterol as a result of higher exposure to air pollution, the effect was stronger for women, the team notes.

Dr Bell said the lower HDL observed with greater air pollution "may put individuals at a higher risk for cardiovascular disease down the line". He added: "This is the first large observational study to suggest an association between air pollution exposure and HDL particle number. This study contributes to the hypothesis that air pollution may act through HDL to contribute to cardiovascular disease at comparably low levels found in developed countries."

Previous studies showed that lower HDL particle numbers were associated with cardiovascular events. The researchers also noted that lower HDL cholesterol is a traditional risk factor for cardiovascular disease.

Although the researchers could not rule out measurement error in their models, they mentioned that measurement error in air pollution estimates was likely independent of HDL measurements. They also noted that air pollution was a mixture of particles and gases, so they might not have measured all pollutants. In addition, they only measured HDL particle numbers once.