While co-exposure exacerbated inflammation and oxidative stress within the lungs, PM exposure was responsible for a substantial portion of that response. The ultrafine particles within PM are capable of reaching terminal bronchioles, embedding themselves in lung tissue, causing the effects described above. These particles will often be phagocytosed, as cells and organelles aid in the clearance of pathogens and other foreign material. The smallest particles can also enter the bloodstream and the brain via the olfactory nerves, promoting endothelial dysfunction and hypertension, as well as potentially triggering atherosclerotic processes via the bloodstream.
On the other hand, the effects of noise pollution were most apparent in the brain, inducing cardiovascular complications as a result of an increase in circulating stress hormones and phagocytic NOX-2 (3, 5). Notably, the vascular and cerebral microvascular impairment induced by aircraft noises was almost completely prevented by NOX-2 knockout and HO-1 activation, demonstrating the importance of these proteins in mediating noise-induced health effects.
The stress response to aircraft noise increased blood pressure by activating the hypothalamic-pituitary-adrenal (HPA)-axis, a mechanism that mediates numerous physiological processes such as metabolism and immunity (4). As isolated noise pollution exposure already substantially increased blood pressure, air pollution may have been precluded from increasing blood pressure further due to physiological limitations.
As an intermediary between many other organ systems, the circulatory system was unsurprisingly affected by both air and noise pollution. Oxidative stress and inflammation from the lungs and circulating stress hormones promoted the development of endothelial dysfunction and hypertension.
This study demonstrates the importance of investigating the individual and simultaneous effects of environmental stressors such as noise and air pollution. When studying these stressors separately, the combined effect of co-exposure is not considered and the health risks of these pollutants may therefore be severely underestimated. Given the global population’s increasing concentration in urban centers, studies such as these are critical to better understanding the potential health impacts of various pollutants. Since particulate matter and aircraft noise are only a subset of the various environmental stressors that affect human health, more holistic approaches such as that presented here are requisites for a comprehensive appreciation of the health impact of environmental factors.