Particulate Air Pollution Correlates with Olfactory Decline in Aging
There is good evidence for particulate air pollution to accelerate degenerative aging. The proposed underlying mechanisms relate to inflammation, as particles can inflame lung tissue, thereby contributing to greater chronic inflammation throughout the body in later life. Separately, declining sense of smell correlates with aging as well, particularly with the incidence of neurodegenerative conditions, likely because similar underlying mechanisms contribute to both. Those mechanisms include the chronic inflammation of aging and its disruptive effects on cell and tissue function. Thereby it is perhaps not surprising to see researchers discover a correlation between olfactory decline and air pollution.
Among sensory dysfunctions, loss in the sense of smell, olfaction, is particularly pronounced in older age. Olfactory deficits are associated with a number of health conditions such as depressive symptoms and frailty, as well as shorter survival. An important fact is that olfactory impairment has exceptionally high prevalence rates among patients with neurodegenerative diseases and may constitute one of the first noncognitive manifestations of an impending dementia.
Given that the olfactory system is directly exposed to the outside environment, it has been speculated that part of the olfactory loss observed in older age may arise from cumulative damage of xenobiotics. For example, an increased exposure to air pollution may lead to olfactory loss, especially among middle-age or older adults for whom xenobiotic exposure has accumulated over a longer time. Sourcing mainly from traffic exhaust and other fuel-burning operations, the smallest particulates, with aerodynamic diameter of less than 2.5μm (PM2.5), are among the most harmful forms of air pollution for human health.
We hypothesized higher exposure to common airborne pollutants to be associated with a faster rate of decline in olfactory identification ability. We tested this hypothesis using a well-characterized population-based sample with spatially detailed data of long-term exposure to air pollution (PM2.5 or NOx) and repeated olfactory identification tests across 12 years of follow-up. Participants showed significant decline in odor identification ability for each year in the study (β=-0.20). After adjustment for all covariates, residents of third (β=-0.09) and fourth (β=-0.07) exposure quartiles of PM2.5 had faster rates of olfactory decline than residents from the first quartile.
Our results suggest an association between air pollution exposure and subsequent olfactory decline. We speculate that cumulative effects of airborne pollutants on the olfactory system may be one underlying cause of olfactory impairment in aging.