Moderate air pollution levels, currently considered to pose little risk to human health, may not be so safe after all.
Two separate studies released this week concluded that inhaling commonplace pollutants can trigger heart attacks or strokes.
One study looked at the big picture. The researchers performed a meta-analysis of 34 previous studies. Every major component of air pollution, with the exception of ozone, was associated with an increased risk of heart attacks, according to their findings published in the Journal of the American Medical Association.
"We found the same results in Europe, the Americas and in Asia," said the lead researcher, Hazrije Mustafic of the Paris Descartes University in France.
"And the majority of studies were done in cities where the air pollution levels were under the recommendations of the World Health Organization," she noted. That suggests pollution generally deemed safe could still have "a deleterious effect."
The second new study focused on the residents of a single American city – Boston, where the air is considered to be relatively clean.
The researchers reviewed the medical records of more than 1,700 stroke patients who were treated at the Beth Israel Deaconess Medical Center over a 10-year period. They matched the onset of stroke symptoms in each patient to hourly air pollution measurements collected at the nearby environmental monitoring station of the Harvard School of Public Health.
"We found that the risk of stroke was about 35 per cent higher on days when the air quality was classified as moderate compared to good days with lower pollution levels," said the lead researcher, Gregory Wellenius at Brown University.
"That is a pretty big jump, especially considering that both these levels – the good days and the moderate days – were below the currently accepted standards for particulate matter pollution set forth by the U.S. Environmental Protection Agency."
Much of the pollution came from traffic, according to the study published in Archives of Internal Medicine.
The researchers said there are numerous ways that foul air could trigger a heart attack or stroke in individuals with pre-existing cardiovascular disease. Past studies have shown for instance that inhaling outdoor air particles can cause inflammation, change how blood vessels work, increase the chances that platelets in the blood will form a clot, and disrupt the autonomic nervous system, which is responsible for controlling heart rate and blood pressure.
Any one of these factors could spell trouble for people with atherosclerosis, or existing plaque in blood vessels. One of these fatty deposits could rupture, creating a clot that blocks blood flow to the heart or brain.
If future research confirms the findings from these new studies, public-health officials may need to revise their pollution standards, said Dr. Wellenius. That could mean tougher measures to curb pollution. Plus, people with cardiovascular disease and lung ailments may be urged to avoid prolonged exposure outdoors on more days of year.