Heightened Risk of Hospitalization for Blood Clots

Association seen regardless of participants’ smoking status and in those with no prior lung disease

(WASHINGTON, Dec. 17, 2024) – People exposed to higher concentrations of three types of air pollution over a period of nearly 17 years had a higher risk of being hospitalized for blood clots in their veins as compared with people exposed to lower concentrations of these air pollutants. The findings were published today in Blood.

“While the increase in risk is modest, it is important because, first, blood clots are an underappreciated condition and, second, air pollution is present everywhere, meaning that even a modest association can result in a large number of blood-clotting events,” said study author Pamela L. Lutsey, PhD, MPH, a professor in the School of Public Health at the University of Minnesota. “Our findings add to accumulating evidence that air pollution contributes to a wide variety of adverse health events.”

Blood clots in the veins, or venous thromboembolism (VTE), are the third most common blood vessel disorder after heart attacks and strokes, affecting about a million Americans every year. There are two types of VTE: deep vein thrombosis, which occurs when a blood clot forms in a vein in the muscles of the arms or legs, and pulmonary embolism, a potentially fatal condition that occurs when a blood clot forms in or travels to the lungs.

Although previous research has suggested that exposure to air pollution may promote the development of VTE, no prior study has had such detailed information about exposure to pollution, and few studies have been done in the United States. Air pollution may promote inflammation in the body, which is thought to increase VTE risk, Dr. Lutsey explained.

The Multi-Ethnic Study of Atherosclerosis (MESA), is a long-term study of the factors leading to the buildup of plaque in the arteries in a racially and ethnically diverse group of over 6,800 men and women living in six communities across the United States. Over time, plaque buildup leads to narrowed arteries, limiting blood flow to the body’s organs and tissues and often leading to cardiovascular disease (heart attacks and strokes). Using the MESA study population, the MESA Air Pollution study, known as MESA Air, was created to see how air pollution affects the development of cardiovascular disease.

MESA Air monitors air pollution in several ways, including long-term monitoring at specific locations, widespread community-based monitoring, and indoor and outdoor monitoring at some participants’ homes. Every two weeks from 2000 to 2018, the MESA investigators combined these data and used them to estimate MESA participants’ exposure to four types of air pollution: fine particulate matter (PM2.5), nitrogen dioxide, nitric oxide, and ozone.

In the study, Dr. Lutsey and her colleagues followed 6,651 MESA Air participants for a median of 16.7 years and collected data on their hospitalizations for VTE. Participants’ average age at study enrollment was 62; 53% were women, 38.4% white, 27.5% Black, 22% Hispanic, and 12% of Chinese ancestry. People taking blood-thinning medications were excluded from the study.

A total of 248 participants were hospitalized for a VTE during the follow-up period. Compared with participants who were at the 25th percentile of air pollution exposure (that is, whose exposure was at the 25th percentile of the whole population’s exposure levels), those at the 75th percentile had a 43% greater risk of hospitalization for a VTE when exposed to fine particulate matter, a 2.8-fold greater risk when exposed to nitrogen dioxide, and a 2.3-fold risk when exposed to nitric oxide. Exposure to ozone was not associated with an increased risk for VTE. Researchers also looked at whether participants’ smoking status (currently, formerly, or never a smoker) and whether evidence of lung disease at enrollment played a role.

“We found that the association of air pollution exposure with an increased risk for VTE persisted regardless of smoking status,” Dr. Lutsey said. “The association was also present in participants who had no prior evidence of lung disease. We believe that the presence of the association in nonsmoking healthy people is additional evidence that air pollution exposure may be associated with increased VTE risk.”

No statistically significant association was found between air pollution exposure and VTE risk in participants who had pre-existing lung disease, she said, but this may have been because the number of study participants with pre-existing lung disease was too small.

A major strength of the study, Dr. Lutsey said, is that it assessed four key components of air pollution, with data updated at two-week intervals for nearly 17 years. In addition, participants were racially and ethnically diverse and lived in geographically dissimilar communities across the United States (Baltimore, Maryland; Chicago, Illinois; Forsyth County, North Carolina; Los Angeles, California; New York, New York; and St. Paul, Minnesota). However, as the study was limited to these six regions, the results may not be generalizable to the entire United States, she said.

Another limitation, Dr. Lutsey said, is that VTE events were identified by the International Classification of Diseases codes recorded when participants were hospitalized and were not confirmed by a review of participants’ medical records. In addition, VTE events that did not result in hospitalization were not counted. Finally, because the study simply observed the effects of air pollution on participants and did not directly compare the effects of exposure to lower and higher levels of air pollution, its findings cannot be interpreted to infer that air pollution caused VTE, she said.

“I do believe, however, that our findings – taken together with other research findings on air pollution exposure and health outcomes – support global efforts to curtail air pollution in order to reduce or prevent a wide variety of diseases,” Dr. Lutsey said.

The study was funded by the National Heart, Lung, and Blood Institute, the National Center for Advancing Translational Sciences, and the Environmental Protection Agency.

Blood (www.bloodjournal.org), the most cited peer-reviewed publication in the field of hematology, is available weekly in print and online. Blood is the flagship journal of the Blood journals portfolio by the American Society of Hematology (ASH) (www.hematology.org).

Claire Whetzel, 202-629-5085
cwhetzel@hematology.org