Scientists have observed that thousands of people die of strokes and heart attacks within 24 hours of a spike in the amount of "microscopic pollution" – particles too small to be filtered by the nose or mouth when breathing – that are produced by vehicle exhausts and coal-burning factories. However, although researchers knew that these particles caused inflammation of the lungs, no one knew how or why they triggered heart attacks.
Now, Mutlu and colleagues have found that the particles, which are smaller than 10 microns, speed up hyperclotting of the blood. This happens because inflamed lungs secrete a substance called interleukin-6, which increases the tendency for blood to coagulate. "This is a critical missing piece of the puzzle that has eluded scientists for decades," says Mutlu, who is a doctor at Northwestern Memorial Hospital. "Now we know how the inflammation in the lungs caused by air pollutants leads to death from cardiovascular disease."
The team obtained its results by mixing particles of pollution, collected by the US Environmental Protection Agency (EPA), into a salt solution and injecting them into the lungs of mice. The blood of mice exposed to the pollution clotted faster than that of mice that had not been exposed. The scientists also observed a 15-fold increase in the amount of interleukin in the blood 24 hours after the animals had been exposed.
Particulate matter pollution is highest near busy roads, motorways and lorry routes. People are exposed when inside a car (even when the windows are closed), on a train or when walking outdoors, explains Mutlu.
Those with a history of heart disease, such as blockages in the coronary or carotid arteries, are at highest risk. "It's important to get screened to see if you have one of these conditions. If so, when there are high levels of particulate matter, you should try to stay indoors and limit your exposure to the outside air," advises Mutlu’s colleague Scott Budinger. Mutlu adds that people at risk may consider taking low-dose aspirin to keep their blood thin.
Exercising near roads also increases the risk because the lungs take in more air. "If you're sitting down, the amount of air you get into your lungs is about five to six litres per minute, but if you're running the amount is 20 to 25 litres," notes Mutlu. "If you're close to an expressway, you're actually breathing more particulate matter into your lungs."
Even more worrying is that a rise in heart attacks and strokes occurs at relatively low levels of particulate matter pollution and that no safe level has yet been established. Mutlu hopes that the new study will encourage the EPA and local regulators to reduce the limits on particulate matter levels.
The risk of dying from a heart attack or stroke increases an enormous 30% with each additional 10 mg of pollution. This is not good news since, according to the American Lung Association State of the Air: 2007 Report, particle pollution appears to be increasing steadily around the US, with many cities exceeding the EPA’s recommended annual average limits. This is probably true for many other parts of the world too.