A new study suggests that ozone might prove to be the key to the link between high temperature and the increased risk of death from heart disease or stroke.
Ozone is chemical pollutant that is strongly tied to weather conditions, particularly the amount of ultraviolet light in the atmosphere.
It is generated by a reaction between nitrogen oxides, volatile organic compounds, and oxygen in sunlight.
The study was based on a population of almost 100 million people in 95 different geographical areas during the summer months of June to September.
The participants were already taking part in National Mortality and Air Pollution Study (NMMAPS), which looked at health and weather pollution between 1987 and 2000.
At this time, 4 million heart attacks or strokes occurred. When the authors compared deaths on daily basis against fluctuations in temperature during one day, they found that ozone was a common link.
The results showed that the higher the ozone level, the higher was the risk of cardiovascular death attributable to high temperatures.
Ozone levels ranged from a daily average of 36.74 parts per billion to 142.85 ppb, while average daily temperatures ranged from 20 to around 42 degrees Centigrade.
A 10 degree temperature increase on the same day was associated with a rise in heart disease or stroke deaths of just over 1percent at the lowest ozone level and by more than 8 percent for the highest levels.
The researchers said that a link between temperature and ozone in driving up cardiovascular mortality was plausible.
They suggested that exposure to ozone might affect the airways and the autonomic nervous system, so making people more susceptible to the effects of fluctuations in temperature.
Scientists said that public health warnings during hot weather ought to include information on ozone levels, when these are high. Rising temperatures and the impact of ozone are likely to become increasingly important as the world heats up as a result of global warming.
The study is published in Occupational and Environmental Medicine.