Why the impact on bones?
The potential negative impact of pollution on bone has been attributed to many factors. PM induces systemic inflammation and an increase in cytokines that stimulate bone cells (osteoclasts) that cause bone loss. Other pollutants (gases and metal compounds) can cause oxidative damage to bone cells, whereas others act as endocrine disrupters and affect the functioning of these cells.
Pollution might also affect the synthesis and metabolism of vitamin D, which is necessary for absorption of calcium from the gut. High rates of pollution can reduce the amount of ultraviolet radiation reaching the earth which is important because certain wavelengths of ultraviolet radiation are necessary for vitamin D synthesis in our skin. Reduced vitamin D synthesis in skin can lead to poorly mineralized bone unless there is sufficient intake of vitamin D in diet or as supplements. Also, the conversion of vitamin D to its active form happens in the kidneys, and PM can be harmful to renal function. PM is also believed to cause increased breakdown of vitamin D into its inactive form.
Conversely, some studies have reported no association between pollution and bone density or osteoporosis risk, and two meta-analyses indicated that the association between the two is inconsistent. Some factors explaining variances in results include the number of individuals included in the study (larger studies are generally considered to be more reproducible), the fact that most studies are cross-sectional and not prospective, many do not control for other factors that might be deleterious to bone, and prediction models for the extent of PM or other exposure may not be completely accurate.
However, another recent meta-analysis reported an increased risk for lower total-body bone density and hip fracture after exposure to air pollution, particularly PM2.5 and nitrogen dioxide, but not to PM10, nitric oxide, or ozone. More studies are needed to confirm, or refute, the association between air pollution and impaired bone health. But accumulating evidence suggests that air pollution very likely has a deleterious effect on bone.
When feasible, it’s important to avoid living or working in areas with poor air quality and high pollution rates. However, this isn’t always possible based on one’s occupation, geography, circumstances, or economic status. Therefore, attention to a cleaner environment is critical at both the individual and the macro level.
As an example of the latter, the city of London extended its ultralow emission zone (ULEZ) farther out of the city in October 2021, and a further expansion is planned to include all of the city’s boroughs in August 2023.
We can do our bit by driving less and walking, biking, or using public transportation more often. We can also turn off the car engine when it’s not running, maintain our vehicles, switch to electric or hand-powered yard equipment, and not burn household garbage and limit backyard fires. We can also switch from gas to solar energy or wind, use efficient appliances and heating, and avoid unnecessary energy use. And we can choose sustainable products when possible.
For optimal bone health, we should remind patients to eat a healthy diet with the requisite amount of protein, calcium, and vitamin D. Vitamin D and calcium supplementation may be necessary for people whose intake of dairy and dairy products is low. Other important strategies to optimize bone health include engaging in healthy physical activity; avoiding smoking or excessive alcohol intake; and treating underlying gastrointestinal, endocrine, or other conditions that can reduce bone density.
Madhusmita Misra, MD, MPH, is the chief of the division of pediatric endocrinology, Mass General for Children; the associate director of the Harvard Catalyst Translation and Clinical Research Center; and the director of the Pediatric Endocrine-Sports Endocrine-Neuroendocrine Lab, Mass General Hospital, Boston.
A version of this article first appeared on Medscape.com.