Air Pollution Linked With Blood Clots

Reuters May 12, 2008

CHICAGO (Reuters) – Air pollution heavy in small particles may cause blood clots in the legs, the same condition air travelers call “economy class syndrome” from immobility during flight, researchers said on Monday. Dr. Andrea Baccarelli of the Harvard School of Public Health in Boston and colleagues said they found the link after looking at 870 people in Italy who had developed deep vein thrombosis between 1995 and 2005. When compared with 1,210 others living in the same region who did not have the problem, they found that for every increase in particulate matter of 10 micrograms per square meter the previous year, the risk of deep vein thrombosis increased by 70 percent. On top of that, the blood of those with higher levels of exposure to particulate matter was quicker to clot when tested at a clinic, they reported in the Archives of Internal Medicine.

Air pollution from automobiles and industry can contain tiny particles of carbon, nitrates, metals and other materials that have been linked over the years to a variety of health problems. While lung diseases were an initial concern, later research has indicated it may cause heart disease and stroke, possibly because it increases the rate at which blood can coagulate, Baccarelli and colleagues said.

Until now particulate pollution had not been linked to blood clots in the veins. The mechanism that causes problems for some air travelers is related not to the blood itself but to impaired circulation when sitting in one place without exercise for long periods of time.

The findings introduce a new and common risk for deep vein thrombosis, the researchers said and “give further substance to the call for tighter standards and continued efforts aimed at reducing the impact of urban air pollutants on human health.”

In a commentary, Dr. Robert Brook of the of the University of Michigan in Ann Arbor said if the findings are proven by additional research it may turn out that “the actual totality of the health burden posed by air pollution, already known to be tremendous, may be even greater than ever anticipated.”

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From Dr. Hasselback:

This case control study from Italy looked at the impact of PM10 levels on the development of deep vein thrombosis (DVT), or blood clots in the legs. The basis for the study were previous observations that persons living in areas with higher PM concentrations had blood changes that would predispose to higher incidence of blood clotting. While the diagnosis of a DVT is fairly definitive and requiring hospitalization, mortality is uncommon and all cases in the study would have had non-lethal events to be recruited.

In short, for each 10 ug/m3 increase in PM10, the risk for DVT increased 70%, which is a larger in comparison to the typical increase seen for other health outcomes. The affect was greater in males, and may in part be related to interaction with known hormonal risks in females that need to be examined in greater depth. The study only looked at PM10 and no data is provided as to whether finer particulate fractions were measured or represent a different risk.

The relationship between venous thrombosis and predisposition to blood clotting, is not the same as arterial thrombosis which would be associated with heart attacks and stroke. So the finding of an impact of PM concentrations on the development of DVT is a relatively new potential health outcome that would have not been explored in previous studies. Caution should always be taken with a first identified relationship such as this and confirmatory studies using different methods will be welcomed before stating that a definitive relationship exists.

Dr. Paul Hasselback, Medical Health Officer, B.C. Interior Health Authority, Kelowna, B.C. 250-868-7818

Ongoing Research

Suspended particulate matter refers to airborne particles that range in size from 0.001 micrometers (µm) to 100 µm (the period at the end of this sentence is about 500 mm in diameter). These particles vary in chemical composition, size and shape, depending on their origin.

PM10 refers to suspended particulate matter less than 10 µm in diameter. Particles larger than 10 µm, such as flyash, settle out close to the emission source relatively soon after being emitted. Particles smaller than 10 µm (PM10) can remain suspended in the atmosphere for long periods of time and travel great distances from the source.

While large suspended particulates, such as flyash, may cause a nuisance

or aesthetic problem, fine particulates (less than PM10) have the greatest effect on human health. Recent health and medical studies have shown that PM10 has extensive and serious human health impacts (Vedal, 1995) – see page 7 for more information on health. There is also evidence suggesting that the smaller the particle, the greater the health risks as these tiny particles are inhaled deeply into the lung cavity.

PM10 is further divided into a coarse and fine fraction, as particles in each fraction generally differ in chemical composition, source and behavior in the air.

  • The fine fraction of PM10 includes particles 2.5 micrometers (µm) in diameter and less. This size fraction is generally composed mainly of secondary particulates, i.e. particles formed from physical and chemical reactions involving gases such as oxides of nitrogen, sulphur and ammonia, and volatile organic compounds (VOCs) that are emitted into the air.
  • Particles less than 2.5 µm are thought to have even greater health impacts than coarser particles because of their ability to penetrate to the deepest regions of our lungs. These particles are also very efficient at scattering/absorbing light and are responsible for regional haze and smog.
  • The coarse fraction of PM10 includes particles between 2.5 and 10 micrometers in diameter. This is the size fraction most often associated with natural sources such as soil particles and fibres.