When temperatures approach or exceed 90 degrees, health officials warn people to stay inside and keep cool, and if that's not possible, drink plenty of fluids, use sunscreen, wear light clothing and avoid exertion.
But a University of Pittsburgh Graduate School of Public Health study has found a 12 percent increase in hospitalizations for stroke when temperatures reach 87 degrees or higher. These are cerebrovascular strokes, not heat strokes. Such strokes, often known as "brain attacks," are caused by blood clots in the brain that can cause paralysis and death.
Temperatures above 87 pose a notably higher risk of stroke for people 65 and older, according to the study led by Pitt epidemiologist Evelyn O. Talbott
and Xiaohui Xu, of the University of Florida College of Medicine.
It documented a 12 percent increase in hospital admissions of Allegheny County residents for stroke. The total 11,207 cases of stroke occurred during the five-year period, 1994 through 2000.
"When it comes to an environment trigger, 12 percent is a significant amount," Ms. Talbott said, noting that heat was the focus but pollution also is a factor. "Two other findings are that men had a higher risk than women, probably because more men spend more time outside, and we also found a greater effect for men and women over 80."
Pollution is another culprit affecting health during hot weather, said Ms. Talbott, who holds a doctorate in epidemiology, and LuAnn L. Brink, also a Pitt epidemiologist.
Ozone and other pollutants probably play a significant role, they said. Previous studies, along with continued research under way, indicate that higher levels of air pollution often associated with hot weather can cause asthma attacks and impact people with cardiovascular and lung diseases.
Using data from the state Department of Environmental Protection, the website airnow.gov reported levels of ozone pollution had risen to become unhealthy for sensitive groups in some areas throughout the region Friday.
Another "action day" is forecast for today, with the ozone level again unhealthy for sensitive groups. Moderate levels of particulate pollution are expected. Sensitive groups include active children and adults, and people with lung disease, such as asthma.
A study involving the two doctors based in Wuhan -- Pittsburgh's sister city in China -- found a 3 percent increase in hospitalizations for strokes from 2006 to 2008 as a result of nitrogen dioxide pollution, the precursor to ozone, and to a lesser degree, particulate pollution.
Ms. Talbott said elevations in ozone pollution and temperature independently cause hospitalizations for asthma. Unhealthy ambient ozone levels represent a notable hot-weather hazard to respiratory function.
"Ozone is very high on hotter days. Clearly ozone affects the lungs and makes them burn," Ms. Talbott said, noting the effects of vascular inflammation. "Less oxygen in the system makes the system work harder, with greater blood coagulation and viscosity, which essentially means the blood is thicker. I really think this is the perfect storm for stroke."
The American Lung Association said ozone, "a highly reactive gas that is a form of oxygen" and commonly known as smog, results in sunburn-like inflammation of the lungs that can cause shortness of breath, chest pain, wheezing and coughing, asthma attacks, and even premature death, with the increased need for medical treatment for asthma, chronic obstructive pulmonary disease, chronic bronchitis and emphysema.