New Delhi: Around 86 per cent of people living in urban areas across the globe, which is almost equal to 2.5 billion people, are exposed to unhealthy particulate matter levels. This resulted in as many as 1.8 million excess deaths in cities globally in 2019, according to a new study published in The Lancet Planetary Health journal.
Also, nitrogen dioxide (NO2) pollution in 2019 resulted in nearly 2 million asthma cases among children worldwide, according to another study published in the same journal. Two of out three cases occurred in urban areas.
PM2.5 Resulted In 1.8 Million Excess Deaths In 2019
In the first study, the researchers focussed on PM2.5, which is fine particulate matter with a diameter of 2.5 micrometres or less.
PM2.5 is the leading environmental risk factor for disease, and its inhalation is known to increase the risk of premature death from conditions such as cardiovascular disease, respiratory disease, lung cancer, and lower respiratory infection.
More than 55 per cent of the world's population lives in cities. Yet, there has been little research into how PM2.5 disease burdens compare across urban areas globally to date. Most assessments analysed PM2.5 in megacities only. In the new study, researchers examined PM2.5 concentrations and associated mortality trends in more than 13,000 cities globally between 2000 and 2019.
There were large variations in PM2.5 by region. The largest regional increases were seen in urban areas in South-East Asia, including India. Between 2000 and 2019, there was a 27 per cent increase in average populated-weighted PM2.5 concentration.
In South-East Asian countries, the PM2.5 attributable mortality rate increased from 33 per cent to 63 per cent to 84 per cent in 100,000 people. These countries witnessed the largest increase in PM2.5 attributable mortality rates, according to the first study.
The World Health Organization (WHO) 2005 guideline for annual average PM2.5 exposure states that the annual average concentrations of PM2.5 should not exceed 10 microgram per cubic metre. Also, 24-hour average exposures should not exceed 25 microgram per cubic metres more than three times a year.
In 2019, around 86 per cent of urban inhabitants worldwide lived in areas exceeding the WHO guideline, which resulted in an excess of 1.8 million deaths, the study said.
Over the two decades, decreasing PM2.5 countries in urban areas did not correspond to the same level of decreases in PM2.5-attributable mortality rates on their own, the researchers found.
For instance, African, European, and North and South American cities experienced 18 per cent, 21 per cent, and 29 per cent decreases in PM2.5 concentrations.
The fact that the decrease in concentrations did not correspond to the same level of decreases in mortality rates attributable to PM2.5 demonstrated that other demographic factors, such as an ageing population and poor general health, are influential drivers of pollution-related mortality burdens, the study said.
Quoting Veronica Southerland of George Washington University, US, and the lead author of the study, a Lancet statement said that the majority of the world's population still live in areas with unhealthy levels of PM2.5. She added that avoiding the large public health burden caused by air pollution will require strategies that not only reduce emissions but also improve overall public health to reduce vulnerability.
The limitations of the study included the fact that some uncertainties in the values were unable to be fully accounted for, such as the use of country-wide baseline disease rates in calculations of mortality. Also, the study only assessed the impact of 2.5 on mortality, without accounting for other health burdens caused by PM2.5, such as low birth weight, premature birth, and cognitive impairment, the authors noted in the study.
Air Pollution In Cities Responsible For Two Thirds Of Paediatric Asthma Cases
In the second study, the researchers focussed on NO2, which is an air pollutant emitted by vehicles, power plants, industrial manufacturing, and agriculture. NO2 serves as a marker for transport-related air pollution. According to previous research, transport-related air pollution is associated with both asthma exacerbation and new onset asthma in children. However, to date, there have been no studies which specifically look at trends in the burden of transport-related NO2 pollution or paediatric asthma incidence in urban areas.
The researchers calculated global NO2 concentrations with a one kilometre resolution by combining satellite data with datasets on different types of land use, like roads and green space.
In order to estimate paediatric asthma incidence attributable to NO2 between 2000 and 2019, in as many as 13,189 urban areas worldwide, the researchers applied the NO2 concentrations to population and baseline asthma rates, the study said.
In 2019, there were 1.85 million new paediatric asthma cases associated with NO2, the researchers found. Also, this accounted for 8.5 per cent of all new cases reported that year.
Two out of three paediatric cases attributable to NO2 occurred in the 13,189 urban areas covered in the study. In 2019, NO2 was responsible for 16 per cent of all new paediatric asthma cases in urban areas.
As many as 1.2 million paediatric asthma cases in urban areas could be attributed to NO2 pollution in both 2000 and 2019, the study said.
However, the rate per 100,000 children decreased from 176 to 156 per 100,000 children. The rate decreased by 11 per cent as the urban population grew by 14 per cent.
Quoting Dr Susan Anenberg of the George Washington University, US, and the first author of the study, the Lancet statement said that the results of the study demonstrate the important influence of combustion-related air pollution on children's health in cities globally. She is also the corresponding author of both studies. Anenberg added that NO2 concentrations have been trending downward for decades in places that have effective air quality management programs, and this has been beneficial for children's respiratory health.
She said that even with these improvements, current NO2 levels contribute substantially to paediatric asthma incidence. This highlighted the fact that mitigating air pollution should be a critical element of children's public health strategies.
The limitations of the study included the fact that baseline paediatric asthma rates may have been underestimated in low- to middle-income countries, leading to an underestimation in NO2 attributable asthma impacts.
Also, due to a lack of data on urban rates, and asthma prevalence within countries, national paediatric asthma rates were used.
Whether paediatric asthma incidence is associated with NO2, the traffic-related air pollution mixture, or the broader combustion-related air pollution mixture is currently not known, the authors noted in the study.