A few bad days of smog due to a temperature inversion might do you no harm, but long-term exposure to air pollution has now been linked to disease. According to a longitudinal study out of Europe, prolonged exposure to dirty air can lead to an increased risk of hypertension, a condition of chronic high blood pressure that damages your vessels, your heart, and can lead to atherosclerosis (inflammation of the arteries), heart attacks, and strokes.

“This is important because hypertension is the most important risk factor for chronic disease and premature mortality,” lead author Barbara Hoffman, a professor of environmental epidemiology at the Centre for Health and Society at Heinrich-Heine University of Dusseldorf, Germany, tells mental_floss.

Prior research had ascertained that “acute changes in air pollution exposure from day to day could raise your blood pressure” in a transient way—that is to say, your blood pressure would rise, but then return to normal. But it was not known if such long-term exposure could lead to the disease of hypertension. The study, published in European Heart Journal, confirms the link between long-term air pollution and increased risk of hypertension. The risk is comparable to the effect of being overweight, Hoffman says.

Of the 41,072 people who participated in the longitudinal study, none had hypertension when they began, but during the follow-up period—at either five or nine years—15 percent had developed hypertension or were taking blood pressure medications. And for people living in the most polluted areas, for every 5 micrograms per cubic meter of pollutants, the risk of hypertension increased by 22 percent over those living in less polluted areas.

Pollution varies from area to area, of course, depending on where you live, which is why the study looked at five different European countries: Norway, Sweden, Denmark, Germany, and Spain. Using land regression statistical models, scientists collected data from 40 sites three times per year for two weeks each period. “Major constituents of pollution in a city include traffic, industrial activity, parts blown in from long-range transport, a mixture of all kinds of things close to you, such as heating of houses, agriculture, and Earth-crust material,” Hoffman says. Agriculture, for example, accounts for a large amount of “precursor gases” that coagulate in the air and form small particles. Hoffman found that overall, southern Europe had higher levels of pollution than the Scandinavian countries.

Hoffman says pollution is thought to cause hypertension by one (or more) of three ways. First, when you inhale pollution particles, they can lead to “pulmonary inflammation, which gives you systemic inflammation,” says Hoffman. “This damages blood vessels and leads to endothelial dysfunction. Arterial stiffness increases, which affects your blood pressure.”

Second, the particles you inhale find their way onto receptors in your lungs that influence your nervous system, particularly the sympathetic nervous system. “This leads to an increase in heart rate, contraction of blood vessels, and a rise in blood pressure. If this happens chronically, you can develop hypertension,” she explains.

Finally, as the pollutant particles directly enter your bloodstream, your blood vessels are damaged “by inflammation, oxidative stress, and can lead to impaired function of the vessel.”

In Europe, the limit value—or how much pollutants are allowed in the air—is 25 micrograms per meter. In the U.S., that number is only 12. Hoffman says, “Our current limit value doesn’t protect the European population.” Hence, the need for this study. “We wanted to inform the European Government, the E.U., about healthy facts at current levels of air pollution. The individual can hardly control chronic air pollution. That’s something society has to take care of.”