Down to the bone
Thanks to Anyonya Guntur for this contribution!
DOWN TO THE BONE
There’s growing evidence that breathing polluted air increases the risk of osteoporosis
Researchers have called osteoporosis a “silent” disease because it often isn’t recognized until a person suffers a debilitating injury. But it is widespread: Globally, an estimated one-third of women and one-fifth of men over age 50 will experience fractures related to osteoporosis. In India, reliable statistics are scarce, but more than 61 million people are believed to have the condition.
Research suggests many factors contribute to osteoporosis, including hormonal changes, a lack of exercise, and drinking alcohol and smoking tobacco. All play a role in India, but so may another factor: exposure to air pollution. Epidemiological studies in numerous nations suggest rates of osteoporosis are higher in areas that have sooty, highly polluted air—for which Indian cities and villages have become notorious. Researchers are still trying to understand the biological mechanisms that might link smog to brittle bones. But, “There is growing recognition that … air pollution is a risk factor for bone health,” says environmental epidemiologist Cathryn Tonne of the Barcelona Institute for Global Health (ISGlobal).
French pathologist Jean Lobstein is credited with coining the term osteoporosis—a combination of the Greek words for bone and hole—in the 1830s. In the decades that followed, scientists unraveled how the balance between bone deposition and absorption goes awry in osteoporosis and identified multiple risk factors. But it wasn’t until 2007 that a study conducted in Norway called attention to a possible connection to air pollution. That study, published in Osteoporosis International, found a weak but statistically significant correlation between modeled exposure to air pollution and reduced bone density in 590 men ages 75 or 76. Three years later, the same researchers found a correlation between air pollution exposure and an increased prevalence of self-reported forearm fractures in men who smoked.
Since then, researchers working in other nations have jumped into the field. In the United States, a team led by Diddier Prada, an environmental and molecular epidemiologist at the Icahn School of Medicine at Mount Sinai, has been probing large data sets assembled by the U.S. government and researchers. One study, published in 2017 in The Lancet Planetary Health, examined a cohort of 9.2 million people above the age of 65 who lived in the northeastern U.S. as well as 692 middle-age men living in low-income communities. In both cohorts, the researchers found that exposure to higher levels of two especially dangerous forms of air pollution—particulate matter less than 2.5 microns in diameter (PM2.5) and soot known as black carbon—was associated with higher rates of bone fractures and other markers of osteoporosis. Last year in eClinical Medicine Prada’s team reported finding an association between osteoporosis markers and exposure to another major air pollutant, nitrogen oxides, among a group of more than 160,000 U.S. women who had already experienced menopause.
In the United Kingdom, an analysis of more than 446,000 participants in the UK Biobank, published in the Journal of Bone and Mineral Research, found that those living in more polluted areas had a 15% greater risk of fractures. Epidemiologist Otavio Ranzani of ISGlobal found a similar pattern in health data collected from 3717 people living in 28 villages near Hyderabad in southern India. Ranzani says his team launched the study, in part, because studies had suggested that “some harmful components from tobacco smoke have similarities with ambient air pollution.” The researchers found individuals living in more polluted villages were more likely to have lower bone mineral content and density, they reported in 2020 in JAMA Network Open.
In China, which has some of the world’s worst air pollution and a rapidly aging population, researchers are finding similar links. One recent study of more than 5000 urban residents of Shandong province, published in the Archives of Osteoporosis, found that even short-term exposure to traffic-related air pollutants appeared to increase the risk of osteoporotic fractures. People living in China’s rural areas also face risks, a 2020 study published in Environmental Research showed. Researchers used satellite, weather, and land use data to estimate the pollution levels experienced by some 8000 residents of a rural area. Even relatively small increases in PM2.5 and other pollutants seemed to increase rates of osteoporosis, they found.
The study’s “substantial sample size” and “robust statistical model” make its results convincing, says environmental health scientist Yuming Guo of Monash University, one of its authors. Prada agrees that the statistical correlations between pollution and osteoporosis are strong. But actually proving causation is difficult, he and other researchers note, because it’s difficult to document exactly how much pollution a person is breathing in—as well as the specific chemicals in the smog. “Individual assessment of exposure is logistically really complicated and quite expensive,” Prada says. It can also be difficult to sift out confounding variables; people who live in highly polluted areas may have other risk factors, such as poor nutrition, that contribute to osteoporosis.
Despite that complexity, researchers have begun to identify some of the ways that air pollutants likely lead to bone damage (see graphic, above). One is in plain sight: Ground-level ozone catalyzed by pollutants can block sunlight, dimming the ultraviolet radiation that enables the body to produce vitamin D, which plays a key role in bone development.
Other potential mechanisms are buried deep within cells. There, compounds found in air pollution can spur the formation of unstable molecules called free radicals that can bind to and damage DNA, proteins, and other cellular components. Pollutants can also promote inflammation and other immune responses, which could interfere with cellular processes that replace old bone tissue with new, Prada’s team noted in 2020 in Environmental Research.
If air pollution does cause osteoporosis, the findings could have major implications in India, where a growing population—now 1.4 billion—is breathing increasingly dirty air. From 1998 to 2021, India’s average annual particulate pollution increased by almost 68%, according to the Energy Policy Institute at the University of Chicago. The burning of fossil fuels is partly to blame, but so is the common practice of burning agricultural residues in rural areas. And many Indians, including Khatave, cook over smoky stoves fueled with dung, firewood, or charcoal. “For more than 25 years now, I cooked at least 4 hours daily on the chuli,” a traditional stove, Khatave says.
Such pollution is already known to take a toll on heart and lung health, but its likely link to osteoporosis provides yet another reason for India to “implement policies leading to [air pollution] reduction,” Ranzani says. The nation also faces a pressing need to improve osteoporosis diagnosis, says Uma Kumar, head of the rheumatology department at the All India Institute of Medical Sciences. Many cases are currently missed, she says, because of a shortage of standard bone density testing equipment, especially machines used to perform dual-energy x-ray absorptiometry, or DEXA, scans. India currently has just about one DEXA scanner for every 4 million people, mostly in big cities. (The scans can also be prohibitively expensive for many Indians.)
In Ganeshwadi, Khatave believes a timely diagnosis could have helped her better manage her deteriorating bone health. Now, she wonders: “I have to rely on painkillers almost every day. But for how long?
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