"Multimorbidity" is defined as the co-existence of two or more long-term conditions such as chronic neurological or respiratory diseases. These diseases may be related to some extent, as clusters (the term "meta-syndrome" is not used but would be handy in this context). The paper, based on UK Biobank data, notes that previous studies in China have identified that very low air pollution is associated with a muscular-skeletal multimorbidity cluster (i.e. people living in rural China become ill from long physical work) and that high air pollution, in particular the smallest resolved particulates, PM2.5, are associated with a cardio-metabolic cluster of diseases.

The present UK study found comparable high levels of harm from PM2.5 (again, the finest category studied) and NO2, with the "neurological" cluster of co-morbidities very interestingly being the most significant category for both major causes, followed by the respiratory group. Cardiovascular, pain-related, and mental health clusters were highly significant for both PM2.5 and NO2, but in different orderings. The neurological cluster of co-morbidities did not explicitly include neurodegenerative disease, possibly because of difficulties in diagnosis, but did include epilepsy which is a common manifestation of neurodegeneration as well as stroke and alcohol/substance dependency. Depression, another typical manifestation of neurodegenerative disease, was one factor in the "mental health" co-morbidity cluster. Combining the neurological and mental-health clusters would create a super-category with very highly augmented risk following either PM2.5 or NO2 chronic exposure.

The large size of the UK biobank data made it possible to control effectively for ethnic and sociodemographic covariates.