Higher levels of environmental pollution correlate with higher incidences of a range of mental disorders, a new study has identified.

The researchers looked at populations in both the US and Denmark. In the US study, the levels of environmental pollution were determined from air, water and land quality measurements, in addition to population density and traffic. The incidences of various mental illnesses were determined within each US county.

Counties with the worst land quality showed a 19.2% increase in diagnosis rate of personality disorder. The worst air quality gave a 27% increase in the incidence of bipolar disorder and a 6% increase in the incidence of depression.

For Denmark, the study identified participant’s levels of air pollution exposure during their childhood. The incidence of schizophrenia was 148% higher for individuals that experienced the highest levels of air pollution, compared to individuals that experienced the lowest. The incidence of bipolar disorder, personality disorder and depression also increased with increasing childhood exposure to air pollution.

It has previously been identified that both genetic and social factors, in addition to environmental factors, can also predispose people to mental illness. The researchers hypothesize that air pollutants could impair cognitive function, possibly through acting on the neuroinflammatory pathways that have been shown to contribute to depression in animal studies.

More research is needed to determine how environmental, social and genetic factors interplay to understand the causes of mental illnesses and develop effective treatments.