According to research, people exposed to air pollution experienced Covid-19 as if they were 10 years older.
The Belgian study also found people recently exposed to dirtier air before contracting the illness spent four days longer in hospital, the same impact as on those 10 years their senior.
It showed that air pollution levels measured in patients' blood were linked to a 36% increase in the risk of needing intensive care treatment. A separate study in Denmark showed air pollution exposure was linked to a 23% increase in the risk of death from Covid-19.
In both studies, the level of air pollution was below legal EU standards.
Previous research suggested that air pollution worsened Covid outcomes but, rather than assessing groups of people together, the new studies followed individual patients, and therefore give much more confidence in the results.
Air pollution is known to be a major risk factor in aggravating respiratory diseases as it increases inflammation in the lungs and weakens immune defences, and causes pre-existing lung problems that worsen the outcomes of new infections.
The new research shows cutting air pollution is a crucial measure for reducing illness and deaths during future outbreaks of respiratory diseases, including annual flu season. It also showed that cleaner air brought health benefits almost as great as some of the medical treatments given to Covid-19 patients.
Professor Tim Nawrot, at Hasselt University in Belgium, said: "Reducing air pollution, even when at relatively low levels, increases the health of the population and makes them less susceptible to future pandemics".
"The pandemic placed an enormous strain on doctors, nurses and other healthcare workers. Our research suggests that air pollution made that burden even greater".
Dr Zorana Jovanovic Anderson, at the University of Copenhagen in Denmark, and Senior Author of the Danish study, said: "These results show how air pollution can compromise our immune system and leave us vulnerable. Reduction of air pollution should be at the heart of preventive measures for current and future pandemics, as well as a strategy for dealing with seasonal influenza pandemics".
The Belgian study, published in the European Respiratory Journal (ERJ), followed more than 300 patients who were hospitalised with Covid-19 between May 2020 and March 2021. Data on the levels of the following three pollutants at the patients' homes were gathered and the amount of soot in the patients' blood was also measured:
Other factors known to affect Covid-19 disease such as age, sex, and weight were taken into account.
The difference in air pollution used in the studies to assess the impact of Covid-19 was based on the range of pollution levels recorded. The higher level used was midway towards the top of the range (75%) and the lower level was towards the bottom of the range (25%).
The researchers found people exposed to the higher level a week before hospital admission went on to spend about four more days in the hospital, and that the lower level of air pollution resulted in health improvements equivalent to 40-80% of the benefits of medicines used to treat Covid.
The Danish study, also published in the ERJ, used Denmark's national Covid-19 surveillance system to follow all 3.7 million people in the country aged 30 or older over the first 14 months of the pandemic. It found patients exposed to the higher level of small particle air pollution in 2019 were approximately 23% more likely to die from Covid-19.
A similar study in New York City found the increased risk of death was 11%.
Professor Charlotte Suppli Ulrik, Head of the European Respiratory Society Assembly on the Environment and Epidemiology, said: "We are finding more and more evidence that breathing polluted air is contributing to lung diseases, including infections. Although the Covid-19 global health emergency is over, the impact of pollution on our health is continuing and we need governments to take action for the sake of our health and our health services".
Professor Jordi Sunyer and Dr Payam Dadvand, at the Barcelona Institute for Global Health, commented on the Belgian study, that: "Despite the evidence, the air quality standards are still above harmful values and even these rather lax standards are still not met in most cities in the world, including many European cities".
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