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Exceeding the World Health Organization (WHO) limits for levels of ozone in the atmosphere is associated with a significant increase in hospitalizations for heart attack, heart failure and stroke, according to a new large Chinese scientific study. But even ozone concentrations below the maximum permissible limit of the WHO are associated with a deterioration in cardiovascular health.

Ozone is a gas that is a major air pollutant and the cause of the photochemical “cloud”. This pollution is a different problem than the hole in the upper ozone layer, which acts as a shield against ultraviolet solar radiation. Ozone air pollution is created when other pollutants react in the air in the presence of sunlight. These other substances are volatile organic compounds and nitrogen oxides emitted by vehicles, power plants, refineries, chemical industries and the burning of biomass (eg wood for fireplaces) and fossil fuels.

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Previous studies have provided evidence that ozone pollution harms the heart and blood vessels. The new research confirms that this is true and highlights the range of cardiovascular risk.

The researchers, led by Professor Shaowei Wu of Jiaotong University, who published in the European Heart Journal of the European Society of Cardiology, correlated data on ozone pollution in 70 Chinese cities (a total of 258 million inhabitants or 18% of the Chinese population), with hospital admissions for cardiovascular problems.

During the three-year study there were 6.44 million hospitalizations for cardiovascular problems, and the average annual peak ozone concentration was estimated at 79.2 µg/m3 (micrograms per cubic meter of air). The WHO upper safety limit is 100 µg/m3. It found that for every 10 µg/m3 increase in ozone levels over a two-day period, there was a 0.4% increase in stroke hospitalizations and a 0.75% increase in acute myocardial infarction hospitalizations.

“Although these increases seem relatively small, it should be noted that ozone levels can even exceed 200 µg/m3 in summer, which means at least a 20-fold increase in imports to over 8% for strokes and 15% for heart attacks,” said Dr. Wan. “During the three-year study and as time went on, ozone was responsible for an increasing proportion of cardiovascular hospitalizations. It is believed that climate change, which creates atmospheric conditions favorable for ozone formation, will continue to increase its concentrations in many parts of the world. Our findings suggest that the elderly are particularly vulnerable to the adverse cardiovascular effects of ozone. This means that worsening ozone pollution due to climate change, combined with the rapid aging of the world’s population, may bring even greater risks of cardiovascular disease in the future,” added the Chinese scientist.

Atmospheric ozone levels below 70 µg/m3 are usually of natural origin and are not associated with any human activity. The Chinese research found that levels do not need to exceed the WHO upper limit of 100 µg/m3 for there to be a health risk. Even levels of 70-99 µg/m3 increase the risk of hospitalization, from 2.3% for heart failure to 3.2% for coronary artery disease. More generally, the Chinese researchers estimated that 3% to 4% of hospitalizations for coronary heart disease, heart failure and stroke were related to ozone pollution.