Air pollution is affecting how well we sleep, according to the findings of an international evidence review. Dr Junxin Li, of the Johns Hopkins University School of Nursing, who led the review, said: “For years our team has been studying sleep in older adults living in the Baltimore area. Outdoor air quality may change block-by-block and, in some residences, we noticed exhaust fumes from nearby traffic. This led us to a simple but critical question: could the air older adults breathe indoors – as well as just outside their front door – be influencing how well they sleep?”
Li’s team searched for studies from around the globe. Focusing on people over 45 years old, they found 25 high quality studies since 2015. These looked at 1.2 million people in six countries including China, India, the US and Germany.
Li said: “The signal was stronger and more widespread than we expected. Across dozens of studies, long-term exposure to outdoor pollutants such as particle pollution, nitrogen dioxide and even carbon dioxide was linked to shorter or lower-quality sleep.”
Some studies measured sleep with wrist-worn devices while others used interviews. They looked at short-term changes in air pollution as well as the impactof air pollution exposure over many years.
Nine of the studies had similar approaches, allowing Li and the team to pool the data to create a single new metastudy. These results can be used to predict the burden from pollution-related sleep problems and benefits from clean air actions in other locations.
Li added: “We can infer that cutting average particle pollution (PM2.5) in half – from typical levels found alongside busy London roads down to the World Health Organization guidelines – could trim the likelihood of poor sleep in middle-aged and older adults by roughly one in 10.”
Older people are more likely to spend a large proportion of their time at home. Despite this, the team found just six high quality studies that looked at indoor air pollution and sleep, the majority of which were conducted in China. People who used solid fuel, such as wood or coal, for heating and cooking were compared with those who used clean fuels.
The solid fuel users had worse sleep quality, including insomnia and short sleep duration. One study found that using an extractor or opening a window during cooking reduced the impacts on sleep.
“Current clean-air policies focus mainly on outdoor sources, but our findings make clear that the agenda must expand,” said Li.
Although wood and coal heater designs vary throughout the world, studies in the UK have found that particle pollution leaks from stoves into homes. Other UK studies have found indoor air pollution from cooking, and especially when cooking with fossil gas.
Li said: “Air quality – both outdoors and inside the home – is an underrecognised contributor to sleep problems in middle and later life. Sleep itself should be treated as a core health indicator when environmental regulations are evaluated, because cleaner air not only safeguards lungs and hearts, it also helps people sleep, supporting cognition, mood and overall resilience later in life.”
