Air pollution studies suggest that almost nowhere on Earth is safe

In a recent study of daily ambient fine particulate matter (PM_2.5) across the globe, Australian and Chinese researchers found that only a tiny portion of the global population is exposed to levels of PM_2.5 below those recommended by the World Health Organization (WHO). The study, published in The Lancet Planetary Health, finds that more than 70% of days globally see levels above what is considered safe.

Fine particles (less than 2.5 µm in diameter) come from motor vehicle exhaust, the burning of fuels by power plants and other industries, and forest and grass fires — but a lack of pollution monitoring stations globally for air pollution has until now meant a lack of data on local, national, regional and global PM_2.5 exposure. The new study, led by Professor Yuming Guo from Monash University, used traditional air quality monitoring observations, satellite-based meteorological and air pollution detectors, and statistical and machine learning methods to more accurately assess PM_2.5 concentrations globally.

“We used an innovative machine learning approach to integrate multiple meteorological and geological information to estimate the global surface-level daily PM_2.5 concentrations at a high spatial resolution of approximately 10 km x 10 km for global grid cells in 2000–2019, focusing on areas above 15 μg/m³, which is considered the safe limit by WHO (the threshold is still arguable),” Guo said.

The study revealed that annual PM_2.5 concentration and high PM_2.5 exposed days in Europe and northern America decreased over the two decades of the study, whereas exposures increased in southern Asia, Australia and New Zealand, and Latin America and the Caribbean. In addition, the study found that:

Despite a slight decrease in high PM_2.5 exposed days globally, by 2019 more than 70% of days still had PM_2.5 concentrations higher than 15 μg/m³.

• In southern Asia and eastern Asia, more than 90% of days had daily PM_2.5 concentrations higher than 15 μg/m³.
• Australia and New Zealand had a marked increase in the number of days with high PM_2.5 concentrations in 2019.
• Globally, the annual average PM_2.5 from 2000 to 2019 was 32.8 µg/m³.
• The highest PM_2.5 concentrations were distributed in the regions of eastern Asia (50 µg/m³) and southern Asia (37.2 µg/m³), followed by northern Africa (30.1 µg/m³).
• Australia and New Zealand (8.5 μg/m³), other regions in Oceania (12.6 μg/m³) and southern America (15.6 μg/m³) had the lowest annual PM_2.5 concentrations.
• Based on the 2021 WHO guideline limit, only 0.18% of the global land area and 0.001% of the global population were exposed to an annual exposure lower than this guideline limit (annual average of 5 μg/m³) in 2019.
   

According to Guo, the unsafe PM_2.5 concentrations also show different seasonal patterns. Northeast China and North India experienced higher levels during their winter months (December, January and February), whereas eastern areas in northern America had high PM_2.5 in the summer months (June, July and August). The team also recorded relatively high PM_2.5 air pollution in August and September in South America and from June to September in sub-Saharan Africa.

Parkinson’s disease risk

Several other studies have recently investigated the health risks of air pollution, with one preliminary study finding that areas of the United States with higher levels of air pollution are associated with an increased risk of Parkinson’s disease. That study is being presented at the American Academy of Neurology’s 75th Annual Meeting, being held from 22–27 April.

The study involved more than 22.5 million people enrolled in Medicare in 2009. Of this group, researchers identified 83,674 people with Parkinson’s disease. Researchers mapped where study participants lived across the US and calculated the rates of Parkinson’s disease for various regions. Researchers also calculated average air pollution exposure levels for study participants by using the zip codes and counties where they lived as well as an air pollution data source on average annual concentrations of fine particulate matter.

Researchers then divided participants into four groups based on average exposure to air pollution. People in the highest exposure group had an average annual exposure of 19 µg/m³ of fine particulate matter, while people in the lowest exposure group had an average annual exposure of 5 µg/m³. In the highest exposure group, 434 new Parkinson’s disease cases developed per every 100,000 people compared to 359 cases in the lowest exposure group.

After adjusting for other factors that could affect the risk of Parkinson’s, such as age, smoking and use of medical care, researchers found an association between Parkinson’s disease and average annual exposure to fine particulate matter, with people in the highest exposure group having a 25% increased risk of Parkinson’s disease compared to people in the lowest exposure group. Researchers found the strongest association in the Rocky Mountain region, which includes Lake County, Colorado, and its surrounding counties. The risk for Parkinson’s disease in those counties increased by 16% when moving up from one level of fine particulate matter exposure to the next level.

“We found a nationwide association between Parkinson’s disease and air pollution exposure, with people exposed to the highest levels of fine particulate matter having an increased risk of Parkinson’s disease compared to people exposed to the lowest levels,” said study author Dr Brittany Krzyzanowski, of the Barrow Neurological Institute.

“We also identified a Parkinson’s disease hot spot in the Mississippi-Ohio River Valley, which is a region that has some of the highest levels of fine particulate matter pollution in the nation,” Krzyzanowski added. The association was however weaker in this area, with only a 4% increase in risk when moving up from one level of fine particulate matter exposure to the next.

“Finding a relatively weaker association where we have some of the highest Parkinson’s disease risks and fine particulate matter levels in the nation is consistent with the threshold effect we observed in our data,” Krzyzanowski said. “In the Mississippi-Ohio River Valley, for example, Parkinson’s disease risk increases with increasing air pollution exposure until about 15 µg/m³ of fine particulate matter, where Parkinson’s disease risk seems to plateau.”

A limitation of the study was that it focuses on fine particulate matter, which contains a variety of airborne pollutants, some of which may be more toxic than others. Krzyzanowski also noted that air pollution is associated with a variety of other health risks, including dementia, that might diminish the likelihood of a Parkinson’s diagnosis, and this may explain the relatively weaker association between Parkinson’s disease and particulate matter in the Mississippi-Ohio River Valley.

Blood pressure

Adolescents’ rapidly growing bodies may be particularly susceptible to long-lasting effects of exposure to air pollutants, including effects on blood pressure. However, most prior studies on air pollution and blood pressure have focused on adults.

To better understand these associations in adolescents, UK researchers led by Alexis Karamanos of King’s College London analysed data collected as part of the Determinants of Adolescent Social Well-Being and Health (DASH) study, which tracks the wellbeing of thousands of ethnically diverse London schoolchildren over time. They used data on 3284 adolescents in DASH to examine associations between blood pressure and exposure to pollution in the form of nitrogen dioxide and PM_2.5; exposures were estimated based on annual mean levels of pollutants where each participant lived. Their results were published in the journal PLOS ONE.

The researchers found that greater estimated exposure to nitrogen dioxide was associated with lower systolic blood pressure, while greater estimated exposure to PM_2.5 was associated with higher systolic blood pressure. These associations were stronger in girls than in boys. No evidence of a relationship between nitrogen dioxide/PM_2.5 and diastolic blood pressure was observed.

For example, a 1 μg/m³ increase in nitrogen dioxide was associated with a 0.30 mmHg (95% CI 0.18 to 0.40) decrease in systolic blood pressure for girls and 0.19 mmHg (95% CI 0.07 to 0.31) decrease in systolic blood pressure for boys. Meanwhile, a 1 μg/m³ increase in PM_2.5 was associated with a 1.34 mmHg (95% CI 0.85 to 1.82) increase in systolic blood pressure for girls and 0.57 mmHg (95% CI 0.04 to 1.03) increase in systolic blood pressure for boys.

The associations between pollutants and blood pressure were consistent regardless of ethnicity, body size or socioeconomic status. That said, four in five (80%) of the adolescents studied were from ethnic minority groups, and the residential estimates suggest that these adolescents were exposed to higher levels of the pollutants than their white peers.

The researchers are now calling for further studies to help confirm and clarify these findings, particularly among young people from different socioeconomic backgrounds. They also note that levels of nitrogen dioxide and PM_2.5 in London remain well above WHO guidelines.

“This longitudinal study provides a unique opportunity to track exposures of adolescents living in deprived neighbourhoods,” said Seeromanie Harding, from King’s College London. “Given that more than 1 million under-18s live in neighbourhoods where air pollution is higher than the recommended health standards, there is an urgent need for more of these studies to gain an in-depth understanding of the threats and opportunities to young people’s development.”

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