Researchers from China and the United States have recently shown that exposure to harmful ozone levels can lead to elevated blood pressure, which is a risk factor for cardiovascular disease.
The researchers - from Duke University in Durham, NC, together with colleagues from Tsinghua University, Duke Kunshan University, and Peking University, all in China - report their findings in the journal JAMA Internal Medicine.
Cardiovascular disease is a group of disorders that affect the heart and blood vessels. It is usually associated with a buildup of fatty deposits in the lining of blood vessels that supply the heart and brain. This buildup is known as atherosclerosis, and it is the main cause of heart attack and stroke.
Worldwide, more people die from cardiovascular disease than from any other cause.
Cardiovascular disease killed 17.7 million people in 2015, accounting for 31 percent of global deaths that year. Of those deaths, "7.4 million were due to coronary heart disease and 6.7 million were due to stroke."
Ozone is a gas made of three atoms of oxygen. It occurs naturally in the upper atmosphere as well as at ground level. Where it occurs in the upper atmosphere, ozone forms a shield that protects us from harmful ultraviolet rays from the sun.
But when ozone occurs at ground level, it is a harmful pollutant produced when oxides of nitrogen (NOx) react with volatile organic compounds (VOCs) in the presence of sunlight.
NOx and VOCs are emitted by motor vehicles and power plants that use fossil fuels, as well as some industrial processes.
Senior author Junfeng Zhang, a professor in global and environmental health at Duke University, says, "We know that ozone can damage the respiratory system, reduce lung function, and cause asthma attacks."
However, he and his co-authors note that while exposure to ozone has been linked to deaths from cardiovascular disease, there is no clear biological understanding of how it affects the cardiovascular system.
Ozone pollution tied to cardiovascular factors
For their study, the researchers monitored 89 healthy adults living in Changsha City in China from December 2014 to January 2015.
They note that, because the participants "spent most of their time in controlled indoor environments," this offered them an ideal setting "for better characterization of air pollutant exposure effects."
Over the study period, the team measured indoor and outdoor ozone levels, as well as other pollutants.
Also, from "spirometry" breathing tests and blood and urine samples that the participants gave at four sessions spread across the study period, the team was able to measure a number of factors that can contribute to cardiovascular and respiratory disease.
When they correlated the participant data to ozone levels, the researchers found that ozone exposure was linked to markers of blood platelet activation and raised blood pressure.
Blood platelet activation is a known risk factor for blood clotting.
'Safe ozone level should be lowered'
The ozone exposure levels at which these links occurred were lower than levels that have been associated with impaired lung function, which is the main basis for the current air quality standard set by the Environmental Protection Agency (EPA) in the U.S.
"This study shows that standards for safe ozone exposure should take into account its effect on cardiovascular disease risk."
Prof. Junfeng Zhang
Prof. Zhang says that in 2015, a third of the U.S. population - that is, around 108 million people - lived in counties wherein ozone levels were higher than the EPA standard.
Global ozone levels are set to rise as the climate gets warmer, so we can expect an "increasing trend with climate change," says Prof. Zhang.
He adds that ozone pollution is difficult to control because the way it is produced in the atmosphere is complex. "For example," he notes, "a reduction in nitrogen oxides does not necessarily mean a reduction in ozone levels."