High temperatures and air pollution may lead to a higher death rate from heart disease or stroke, scientists have said.

Research in the U.S. revealed that ozone might be the link between high temperatures and the increased risk of death from heart disease or stroke, the British Medical Journal reported.

Ozone is a form of oxygen, which acts as a shield against harmful ultraviolet rays from the Sun, if it is high in the atmosphere. If it is on ground level, ozone can cause lung damage and trigger asthma attacks.

The results of the study will be published in the Occupational and Environmental Medicine journal. The research was undertaken on a population of nearly 100 million people in 95 different U.S. geographical areas from June to September, the period with the highest levels of temperature.

The participants in the study were already taking part in the National Mortality and Air Pollution Study, which is an examination of health and weather pollution between 1987 and 2000.

It is already a fact that higher temperatures are harmful to health by placing the body under stress. More than 27,000 people died in Europe, including 2,000 in Britain during the 2003 summer heat wave.

Over 4 million heart attacks or strokes occurred last year, according to the study. The authors discovered that as ozone levels rose, so did the risk of heart disease or stroke death attributable to high temperatures.

Average daily temperatures ranged from 20 to 40 degrees, while ozone levels ranged from daily average of 36.74 ppb (parts per billion) to 142.85 ppb. A 10-degree temperature rise within one day was correlated to a 1 percent rise in heart disease or stroke at the lowest levels of ozone. The percentage rises to eight units for the highest ozone level.

Being exposed to ozone, people are more susceptible to the effects of variations in temperature, the researchers from the University of California Irvine said. They recommended that public health warnings should include details on ozone levels when temperatures are high.

The authors also warned that people would be more exposed to high temperatures and to the impact of ozone in the futures, as the world would heat up because of global warming.