The heart doesn’t pound with fear or race with excitement because a person wants it to. These things happen involuntarily, triggered by commands from the autonomic nervous system. A new study reports that breathing high concentrations of fine dust elicits subtle changes in autonomic control of heart rhythms.
The finding suggests why certain dust-generating jobs may increase a worker’s risk of cardiovascular disease, including heart attacks.
David C. Christiani and his colleagues at the Harvard School of Public Health in Boston recruited 40 healthy men to wear devices that continuously recorded their heartbeats on several days. All were boilermakers–some apprentice welders and others seasoned journeyman–assigned to the repair of massive electric-utility boilers.
At work, each encountered welding fumes or soot containing high concentrations of particles 2.5 micrometers in diameter or smaller. Such so-called PM-2.5 pollutants can be inhaled deeply into lungs. Heart-rate measurements continued when the men were off work, even on weekends. This enabled the researchers to evaluate the men’s heart responses to a range of stresses, exertions, and concentrations of particulates.
When exposed to workday PM-2.5 concentrations, which were often more than quadruple those in Boston’s outdoor air, the men’s heart-rate variability fell; diminished heart-rate variability is a risk factor for heart attacks.
Everyone’s heart rate fluctuates from moment to moment in response to stress, breathing rate, and other factors, yet people differ in the range that this rate varies. One individual’s heart rate may fluctuate between 59 and 61 beats per minute (bpm), while another’s may swing between 50 and 70 bpm.
The boilermakers tended to experience their lowest variability at about 1 p.m. on work days, even though PM-2.5 pollution might have peaked earlier, according to a report in the Aug. 28 Circulation. The low variability persisted until the end of the men’s 12-hour shifts, usually 5 or 6 hours later, Christiani says.
Overall, he told Science News, the team saw a significant decrease in heart-rate variability as PM-2.5 pollution increased. Though the occupational exposure was much higher than the public would encounter in most urban areas of the United States, Christiani notes, “some Asian cities are easily that [polluted].”
These findings dovetail with data recorded in an elderly population. Last year, researchers at Harvard and at Beth Israel Deaconess Medical Center, also in Boston, reported finding “short-term autonomic imbalance, reflected by changes in heart rate and heart-rate variability.” Short-term tests in 21 people showed lower heart-rate variability on days when outdoor PM-2.5
pollution was highest.
What distinguishes the new study, besides the higher dust concentrations, is that the researchers recorded heart patterns for longer periods on each test day and then averaged data over different stretches of time. In doing so, the latest study uncovered evidence of two different effects of dust.
One emerges almost immediately after breathing heavy dust. Its characteristics suggest that particulate intake affects a person’s autonomic nervous system, Christiani says. The second effect shows up only after several hours of breathing workplace dust and hints of heart-rate changes due to inflammation.
“We have long known that air pollution is bad, not just for the lungs,” notes Antonio Sastre of the Midwest Research Institute in Kansas City, Mo. “But how to link effects directly to the cardiovascular system has been a little mysterious.”
If confirmed, the new study’s hint at a linkage between heart-rate variability and dust-induced inflammation could indicate that “you receive small but constant damage whenever you’re exposed,” Sastre says. “Prior to this study, you wouldn’t have had a reason to look for that.”