MY WORLD OF TRUTH

As researchers probe the physiology underlying noise's cardiovascular consequences, they are zeroing in on a culprit: dramatic changes to the endothelium, the inner lining of arteries and blood vessels. This lining can go from a healthy state to one that's "activated", and inflamed, with potentially serious ramifications.

The path from noise to blood vessels goes something like this: when sound reaches the brain, it activates two important regions – the auditory cortex, which interprets noise, and the amygdala, which manages emotional responses to it. As noise gets louder, and especially during sleep, the amygdala activates the body's flight-or-fight response – even if the person isn't aware of it.

Once initiated, this stress response releases hormones such as adrenaline and cortisol into the body. Some arteries constrict, others dilate, blood pressure rises, digestion slows while sugars and fats flood the bloodstream for quick use by the muscles.

The cascading stress response also prompts the creation of harmful molecules that cause oxidative stress and inflammation in the lining of blood vessels. This dysfunctional endothelium meddles with blood flow and affects numerous other processes that, when impaired, contribute to a range of cardiovascular illnesses, including high blood pressure, plaque build-up in the arteries, obesity and diabetes.

Studies on people and mice show that the endothelium doesn't work as well after just a few days of night time airplane noise exposure, suggesting that loud noise isn't a concern only for people already at risk for heart and metabolic problems. Healthy adults subjected to train recordings as they slumbered had impaired blood vessel function almost immediately, according to a 2019 study by Münzel and his colleagues.

"We were surprised that young people, after hearing these sounds for just one night, had endothelial dysfunction," says Münzel, who also co-authored a review of noise and cardiovascular health. "We always thought this was something that takes years to develop."

While the data continues to accumulate, untangling cause and effect can be tricky. It's not easy to conduct long-term sleep experiments or to distinguish between the effects of daytime and night time noise, or effects of the noise itself versus the combined effects of noise and air pollution (which often go hand-in-hand).

The consequences of environmental noise are also tough to parse due to the subjective nature of sound, says Andreas Xyrichis, a health services scientist at King's College in London. Xyrichis studies hospital intensive care units, where ringing telephones and clattering food dishes can be comforting or counteractive to recovery, depending on the patient. "We are really trying to make this distinction between decibel levels and perception of noise," he says. (Learn more about the effect noise has on hospital patients.)

But despite the remaining questions, there is a growing recognition of the connections between noise pollution and reduced physical health. A 2018 report by the WHO noted that each year, western Europeans are collectively losing more than 1.6 million years of healthy life because of traffic noise. This calculation is based on the number of premature deaths caused directly by noise exposure as well as the years lived with noise-induced disability or illness.

And that number is likely to grow. In 2018, 55%of people lived in cities, and by 2050 that count is expected to reach nearly 70%, the United Nations estimates.

Some governments, heeding public protests, have tried to quiet the clamour of urbanisation by adopting night time flight bans, incentivising quieter technologies and issuing fines for noise complaints. Individuals can help themselves by ensuring that bedrooms are as quiet as possible by retrofitting windows or hanging noise-reducing curtains or, if they can afford it, moving to quieter neighbourhoods.

Cheaper solutions may be wearing earplugs at night or moving bedrooms to a quieter part of the house, according to Mathias Basner, a psychiatrist and epidemiologist at the University of Pennsylvania and president of the International Commission on the Biological Effects of Noise. He believes people should take such steps even if they don't find themselves especially fazed by noise.

"If you're living in Manhattan, you won't notice how loud it is after a while because it is normal," he says. "But if you have habituated to it psychologically, that doesn't mean it doesn't have negative health consequences."