The number of health effects linked to air pollution keeps growing. We already know dirty air is associated with problems in the lungs, heart, uterus and eyes and could potentially affect mental health – and now weaker bones can be added to the list.
Researchers took readings of levels of PM2.5, a fine particulate form of pollution, at 23 sites outside Hyderabad in India. Then they worked with more than 3700 people – with an average age of 35.7 – in nearby villages to explore whether exposure to the air pollution was correlated with changes in the bone mineral content of their hips and spines, a measure of bone strength used to diagnose osteoporosis.
“What we see overall is a quite consistent pattern of lower bone mineral content with increasing levels of air pollution,” says Cathryn Tonne at the Barcelona Institute for Global Health.
People in the area were exposed to average PM2.5 annual levels of 32.8 micrograms per cubic metre. This is three times higher than the safe limit recognised by the World Health Organization. After adjusting for other possible factors – including wealth – Tonne and colleagues found every extra 3 micrograms per cubic metre of PM2.5 was associated with an average reduction in bone mineral density for both men and women of 0.011 grams per square centimetre in the spine, and 0.004 g/cm2 in the hip. Black carbon, a subset of PM2.5, was also associated with lower bone mass.
More than half of the people in the study live in homes where food is cooked using solid biomass fuels, which release the pollutants. But no link to bone mass was found for those who used biomass as their main cooking fuel, and would have been exposed to indoor air pollution from it. This suggests it is the general exposure to air pollution in the ambient air that is responsible for the link.
Research linking air pollution and bone mass is still in its infancy, and most has focused on older people in richer countries. Tonne says previous studies’ findings have been inconclusive, though some have also found associations, such as a 2017 study of older men in Boston.
“The scientific literature on air pollution and bone health is very scarce,” says Tuan Nguyen at the Garvan Institute of Medical Research in Australia. For that reason, he says the new study is important. “It provides a clear and growing evidence that air pollution adverse affect bone health in young adults.”
The fact that the researchers looked at both indoor and outdoor pollution exposure, and studied a large number of people, gives weight to the research, says Frank Kelly at King’s College London. “It’s yet another paper linking exposure to air pollution with a health effect,” he says.
Public health burden
The study suggests the average impact on bone mass is relatively small for individuals, says Cecilie Dahl at the University of Oslo. “It is difficult to say how serious the health impact is on bone strength,” she says. But she adds that the decreases in bone mass can stack up to a significant public health burden at a population level.
To give an idea of how that can play out, Diddier Prada at Harvard University, one of the team behind the Boston study, says that in the US, PM2.5 pollution could be attributed to more than 86,000 osteoporosis-related bone fractures a year. Most people also achieve peak bone mass between 20 and 30 years old, so a deficit in that peak due to air pollution also increases their risk of osteoporosis when they are older.
Possible candidates for how air pollution affects bone mass include inflammation and an imbalance in the body between free radicals and antioxidants. Tonne says more research is still needed on the link between air pollution and bone health, in the shape of bigger studies done over time. “It’s a body of literature that will really give us a good idea of what’s going on, not just one study,” she says.
Journal reference: JAMA Network Open, DOI: 10.1001/jamanetworkopen.2019.18504