Scientists at the Alfred Wegener Institute for Polar and Marine Research observed the highest air pollution on record above the Arctic Circle in May. Air pollution has been linked to a number of medical problems including heart attacks, asthma and stroke. 

It is so hot in many parts of the U.S. that officials are cautioning people to stay indoors.  Even short exposure to high temperatures can cause serious health problems.

Larry Kalkstein

Professor Larry Kalkstein from the Center for Climatic Research at the University of Delaware says, "Many more people die of the heat in New York, Philadelphia and Chicago than they do in Phoenix, New Orleans or Miami, because they are not used to the heat up there."

Heat is not the only danger factor. Hot weather with little or no wind can lead to high levels of air pollution, especially ozone. The elderly and people with respiratory or pulmonary disorders are especially at risk.

Last year researchers in Boston analyzed pollution rates in nine major U.S. cities. They found the risk of stroke was one percent higher on days with relatively higher air pollution.  Scientists say while this increase may seem small, it has a huge effect, since the number of people living in pollution-prone cities is so great.

It is not known exactly how pollution affects our bodies or how it enters the brain. Researchers say pollution particles in the air may enter the body through the lungs and irritate the walls of blood vessels, encouraging clots that travel to the brain.

The exhaust from this train adds to the pollution particles in the air

Scientists at Johns Hopkins University have recently shown a direct relationship between fine particle air pollution and risk for hospitalization from heart attack and respiratory diseases.  The Hopkins scientists found these fine particles can reach the small airways and the air sacks in the lungs.

A study from the Harvard School of Public Health found a link between fine particles in air pollution and risk of death.  It also found that reducing exposure to air pollution decreased the number of deaths associated with pollution.

The next step for scientists -- finding out which specific components of air pollution are the most toxic.