The world is increasingly bad for our lungs
Pollution, climate change and certain familiar pathogens are chasing each other into a downward spiral of risk for respiratory health.
- 19 December 2023
- 9 min read
- by T V Padma
India's capital Delhi has once again won top honours in the State of Global Air (SoGA)'s list of the world's most polluted cities for 2023, and not for nothing. Breathing severely polluted air has become a regular winter feature for the city's residents, with a heavy smog descending on the city around November each year.
"Basically, mortality related to heat stress is doubled if you have high air pollution compared to a low air pollution situation."
– Kristen Aunan, Centre for International Climate Research
There are several reasons. As with most polluted cities worldwide, Delhi grapples with increasing vehicle emissions. To add to the problem, in November, winds usher in soot from crop fires in the neighbouring breadbasket states of Punjab and Haryana, where traditionally farmers burn the crop stubble after harvest. It is worsened again by heavier traffic and fireworks during the traditional festive season around the same time.
The polluting particles are generally classified in two broad categories – inhalable particulate matter less than 10 microns in seize (PM10) and the more detrimental particles less than 2.5 microns in size (PM2.5). Since daily monitoring began in 2018, Delhi has recorded PM2.5 peaks rated "severe" or "severe+" – the most extreme rating – each winter.
Delhi is symbolic of the rising air pollution worldwide that is threatening people's health, one of the topics discussed at the health pavilion at the recently concluded 28th conference of parties of the United Nations Framework Convention on Climate Change (UNFCC) in Dubai.
Deadly air
The SoGA report cautions that air pollution is responsible for one in nine deaths worldwide and accounts for 6.7 million deaths globally in 2019 alone. Of these, more than four million deaths were linked to exposure to outdoor fine particle pollution worldwide. Breathing polluted air increases a person's risk of heart disease, lung diseases, asthma and respiratory infections.
The World Health Organization (WHO) estimates that every year, seven million people die prematurely from exposure to air pollution. WHO data shows that "almost all of the global population (99%) breathe air that exceeds the WHO guideline limits, and contains high levels of pollutants, with low- and middle-income countries suffering from the highest exposures."
And the World Bank estimates that the global cost of health damages associated with exposure to air pollution is $8.1 trillion, equivalent to 6.1% of global GDP.
Global warming is making matters worse. At COP28 this month, a sideline event addressed the inter-linkages between climate change, air pollution and health. Citing an analysis of heat stress-related deaths due to global warming, in several European cities, Kristin Aunan, research director at the Center for International Climate Research (CICERO), Norway, reported initial findings from a pan-European study spanning 854 cities on how air pollution amplifies the effects of heat stress. "Most people will die of lung diseases in our cities when high temperatures are combined with high levels of air pollution," she said.
Death rates increase by 10% as one goes from areas with moderate temperatures to those with high temperatures, she reported. Deaths are higher in people aged 65 or more. "Basically, mortality related to heat stress is doubled if you have high air pollution compared to a low air pollution situation," she said, adding that reducing air pollution is consequently an adaptation measure for heat stress.
Co-exposure to heat and air pollution in LMIC (low- and middle-income country) settings is an unexplored issue with potentially large impacts on health and economy.
Vulnerable cohorts
The people most at risk to air pollution include children, pregnant people and their developing foetus, older adults, and people already with a history of asthma, heart disease and diabetes.
Traffic pollution has an impact on air quality over a large area, but people living near busy roadways are most critically exposed to a mix of harmful pollutants. These include nitrogen oxides, particle pollution and so-called 'volatile organic compounds' or VOCs, a term that describes chemicals that can evaporate quickly, which come from the exhaust pipes of vehicles as well as from the wear of brakes and tyres.
Similarly, at higher risk are people who work outdoors, like construction workers, or vendors selling their produce in small roadside carts, because of the long hours of exposure on bad air days. Many outdoor workers in developing countries are poor, and have limited options for reducing their exposure without jeopardising their employment. If they are engaged in strenuous activity – construction workers, for instance – it increases their breathing rate and the amount of polluted air they inhale.
Damaging growing lungs
One of the most vulnerable groups are infants. WHO data shows that the combined effects of ambient air pollution and household air pollution is linked to 6.7 million premature deaths annually, and 4.2 million premature deaths worldwide in 2019. Some 89% of those premature deaths occurred in low- and middle-income countries, and the greatest number in the WHO South-East Asia and Western Pacific Regions.
"Evidence is still emerging, but it is very likely with strong biological plausibility that air pollution has a long-term impact on infant and child lung health," says Professor Steve Graham, senior technical advisor, International Union Against Tuberculosis and Lung Disease.
"Exposure to severe air pollution is associated with increased frequency and severity of common childhood lung diseases, such as pneumonia and asthma," says Graham.
"Severe lung disease in infants and children can have short-term and long-term impact on lung health. Lung function can remain diminished even after recovery from the episode and there may be an increased risk of developing chronic lung disease as an older adult, which impacts quality of life and life expectancy," he adds.
Children are more vulnerable to polluted air than adults because their airways are smaller and still developing, according to the American Lung Association. They also breathe more rapidly and inhale more air relative to their size than do adults. Their immune systems are still developing – the body defences that help adults fight off infections are still not fully in place.
The association says that children tend to develop more respiratory infections than adults, which increases their susceptibility to air pollution.
Kids are also more likely to spend time being active outdoors, which can increase their exposure on bad air days, and are closer to the ground or face level when it comes to vehicle exhausts. "Growing up breathing high levels of air pollution can affect how children's lungs develop, putting them at greater risk of lung disease as they age," the ALA says.
Exposure to toxic pollutants over a longer period of time puts children at risk of poor lung health and function as they grow older; wheezing, coughs, pneumonia, and even increased risk of lung cancer later in life, according to the organisation Asthma + Lung UK.
At particular risk are babies born to women exposed to severely polluted air during pregnancy. Being exposed to air pollution in the womb can affect a baby's lung development, says Asthma + Lung UK.
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Exposure to air pollutants during pregnancy can also lead to an increased risk of infants being born too small, with low birth weights, or too early.
Pre-term babies are more heavily impacted. A study published in the American Journal of Respiratory and Critical Care Medicine in 2022 reports that preterm infants "showed significantly higher susceptibility even to low to moderate prenatal air pollution exposure than term infants, leading to increased impairment of postnatal lung function."
Polluted air has also been linked to asthma and lower respiratory infections among children, the report says. And research indicates that the effects could last into adolescence. For example, a 2019 report in Environmental International by scientists from Germany, Poland and Australia, says that exposure to air pollution during infancy affects lung function growth up to adolescence.
Impacts on TB, COVID-19 outcomes
Also garnering interest is the impact on the outcomes of tuberculosis and COVID-19 in patients living in severely polluted areas.
"TB-endemic regions of the world have some of the worst air pollution," points out Crystal North, a pulmonary and critical care physician at Massachusetts General Hospital, and Assistant Professor at Harvard Medical School. Air pollution exposure increases the risk of tuberculosis, but the mechanisms underlying these relationships are likely to involve several mechanisms, she explains.
In general, since indoor and outdoor air pollution tend to be highest in regions where people live in more crowded living situations, "at least part of the risk comes from people being in crowded conditions, which increases risk of TB transmission," explains North. Higher outdoor air pollution levels often cause people to congregate indoors, which may increase risk of tuberculosis transmission.
Air pollution has also been shown to directly cause cellular toxicity. In laboratory studies, ambient air pollution (PM10 and PM2.5) from Mexico City was found to decrease lung cells' ability to protect against tuberculosis infection and/or fight off tuberculosis once infected.
"Evidence is still emerging, but it is very likely with strong biological plausibility that air pollution has a long-term impact on infant and child lung health."
– Professor Steve Graham, International Union Against Tuberculosis and Lung Disease.
Similarly, a study that explored the association between outdoor PM2.5 concentration and the seasonality of tuberculosis for Beijing and Hong Kong concluded that higher ambient PM2.5 levels in winter were associated with increased TB cases in the following spring or summer. "So there is some literature to suggest that the pollutant exposure itself is a risk for tuberculosis," says North.
There are an estimated 155 million people who have survived TB globally. Post-tuberculosis lung disease is an increasingly recognised after-effect of TB infection that may be present in more than half of TB survivors. "Indoor and ambient air pollution have well-established negative effects on a variety of acute and chronic lung diseases, so it's quite likely that air pollution exposure also worsens post-tuberculosis lung disease," North adds.
Researchers are, however, yet to publish concrete evidence charting the effect of pollution on the long-standing problem of tuberculosis, and the more recent one of COVID-19 infections. WHO says that long term exposure to air pollution affects the immune system and, therefore, increases susceptibility to any type of respiratory diseases, as well as to chronic diseases such as heart diseases and diabetes. These, in turn, are co-morbidities understood to increase the risk of severe COVID-19.
Anurag Agrawal, Dean of Biosciences and Health Research at Ashoka University near Delhi, who previously led India's national genome sequencing programme for the COVID-19 pandemic, says that linkages between air pollution's impact on COVID-19 are unclear. Agrawal points out that COVID-19 infections are fewer in much of east and south-east Asia, which are relatively more polluted than Europe or the US. He surmises that if COVID-19 was more severe in people with lungs affected by pollution, it should also have proved severe in smokers – but strong evidence for that theory is lacking. "There is no good analysis that accounts for all the confounders."
Indeed, a review by researchers in Italy and Greece of 16 studies reporting 355 separate pollutant-COVID-19 estimates, published in 2022, reports that "air pollution may be associated with worse COVID-19 outcomes," but "research is needed to better test the air pollution-COVID-19 hypothesis, particularly using more robust study designs and COVID-19 measures that are less prone to measurement error and by considering co-pollutant interactions."
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