Research into new questions about links between mental health and air quality has found an association between levels of anxiety and levels of fine particulate air pollution. A second paper in the same issue of The BMJ answers more established questions about links with stroke, too.

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More recent exposure to fine-particle air pollution was linked to anxiety, as was distance from a major road.

Using information from the large amounts of data collecting in the 2004 Nurses’ Health Study, the researchers looked back for a link to anxiety from an estimate of exposure to air pollution among the 71,271 women.

Residential proximity to a major road and average past exposures to fine and coarse particulate matter were modeled and found to be associated with increased anxiety symptoms.

Among the nurses who responded to the questionnaire, 15% experienced high levels of the following symptoms:

  • Fearfulness
  • Desire for avoidance
  • Tendency to worry.

This is similar to the level in the general population cited by the publishers of the study: “Anxiety is the most common psychiatric disorder and globally affects around 16% of people at some point in life.”

The study, by Melinda Power, ScD, and co-authors, found a significantly increased odds of increased anxiety being found in women who had higher exposures to air pollution – specifically, to the fine particulate type.

No link was observed for coarse particulate air pollution, however. (The study examined two levels of particulate matter – fine particles, less than 2.5 microns in size – PM <2.5 μm - or coarse particles, PM 2.5-10.0 μm.)

Women who lived between 50 and 200 meters from a major road were more likely to have higher anxiety symptoms than women living either further away from or closer to a major road. The link to anxiety was not observed for distances of over 200 m or for women living within 50 m of a major road.

The anxiety association was dependent on levels of exposure and was found to be stronger if the exposure to air pollution was recent.

The link to particulates was stronger a month immediately before the anxiety scores, compared with a weaker association against the longer-term measures of exposure at 3 months, 6 months, 1 year and 15 years.

An editorial accompanying the anxiety study, which also comments on the stroke study, cites a possible biological explanation for the association with mental health: “Since air pollution causes systemic inflammation, it is reasonable that researchers have now turned to the arena of mental health, a leading priority for research given the relative absence of known modifiable risk factors and a high and growing disease burden,” writes Prof. Michael Brauer, ScD, from the University of British Columbia’s public health school in Vancouver, Canada.

In the second paper looking into a better-established field of research on health effects of air pollution, the authors report on a systematic review and meta-analysis of over 100 observational studies covering 28 countries around the world to establish a link between short-term air pollution exposure and stroke-related hospital admissions and deaths.

Prof. Brauer says:

The role of air pollution as a possible trigger for stroke has important implications for disease burden, especially in China where air pollution and the incidence of (especially haemorrhagic) stroke are high.”

The researchers from Edinburgh University in the UK looked across the studies and compared risks of admission or death from stroke with levels of gaseous pollutants – carbon monoxide, sulphur dioxide, nitrogen dioxide and ozone – and particulate matter air pollution, both fine particles less than 2.5 microns and coarse particles less than 10 microns.

They found the stroke rates were increased for both PM2.5 and PM10 – with risk rising, respectively, by 1.1% for every 10 micrograms per meter cubed of air, and by 0.3% per 10 micrograms/m3.

For gaseous pollutants, the associations with stroke risk were:

  • Carbon monoxide increasing the risk by 1.5% for every 1 part per million (ppm) increase
  • Sulphur dioxide – 1.9% increase per 10 parts per billion (ppb)
  • Nitrogen dioxide – 1.4% per 10 ppb.

Low- to middle-income countries experienced the strongest associations compared with high-income countries, and the results suggest a need for policy changes to reduce exposure in highly polluted regions such as China, conclude the authors.

Prof. Brauer comments in the editorial: “The impact of chronic exposure to air pollution on development of carotid atherosclerosis (a precursor for stroke) remains unclear. Although this is not covered in the analysis, evidence of an association is growing.”