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expert reaction to short term exposure to fine particulate matter and hospital admission risks

Research, published in The BMJ, reports that short term exposure to fine particular matter in the air is related to newly identified causes of hospital admissions. 


Dr Stefan Reis, Science Area Head for Atmospheric Chemistry and Effects, Centre for Ecology & Hydrology (CEH), said:

These results add to the growing body of scientific evidence showing adverse health effects of air pollutants below current air quality policy target values.

“As we increase our understanding of the mechanisms how air pollution affects human health on a systemic level, results like this highlight the need to revisit air quality guideline and target values. They have been established in a science-policy dialogue as achievable policy objectives to reduce public health effects, but cannot be seen as ‘no effect’ thresholds. In this context, the findings of this study stress once more that policies to reduce overall population exposure to air pollution per se should be the focus, not the attainment of current legal limit values at a small number of official monitoring sites across cities. Finally, the findings suggest that interventions reducing pollution levels even in areas where they are already low will contribute to reduced public health impacts and costs.”


Dr Suzanne Bartington, Clinical Research Fellow and Honorary Consultant in Public Health, University of Birmingham, said:

“The study published by Wei and colleagues in the BMJ utilises a large Medicare database to assess risk of hospital admission associated with fine Particulate Matter  (PM2.5) exposure for specific disease groups among the elderly US population. The links between short-term PM2.5 exposure and increased risk of hospitalisation for respiratory and cardiovascular disease are well established; however this investigation extends our current understanding to a range of new causes, generating findings which contributes to existing knowledge about the population health impacts of air pollution. Specifically the authors found that each 1 μg/m3 increase in ambient PM2.5 was associated with 2050 extra hospital admissions, 12 216 days in hospital, and $31m (£24m, €28m) in care costs, through diseases not previously associated with PM2.5 Importantly, with regard to policy, these associations were observed at daily PM2.5 levels below the existing WHO 24-hour guideline (25 µg/m3). 

“This is a large-scale study conducted using claims records of over 95-million US Medicare recipients aged over 65 years, from 2000-2012. The study population includes those who are already known to be among the most vulnerable to air pollution (elderly people). Although a large sample size, there are some limitations with use of this study population to make inferences about the general population; the Medicare population is recognised to have a higher proportion of those on low incomes and ethnic minority groups and will inherently introduce selection bias. This requires consideration if findings are to be reliably extrapolated to other settings and contexts.  

“As with all retrospective studies which rely upon data collected for another purpose, it is important to consider the accuracy of coding for specific causes of hospital admissions, and how this may have changed over the time period of the study. Furthermore, many conditions will have  overlapping and interrelated aetiological and risk factors, for which are is not possible to investigate in any level of detail regarding independent associations, using this approach.

“The findings concerning risk of admission for cardiovascular and respiratory diseases in relation to short-term PM2.5 exposure are broadly consistent with existing epidemiological evidence. The reported burden associated with newly associated conditions (septicaemia, fluid and electrolyte disorders, renal failure, infections of the skin and subcutaneous tissue) provide novel evidence concerning the contribution of air pollution to disease morbidity. It is challenging to apply condition-specific risk estimates as some of these conditions will share similar PM2.5 pathogenic mechanisms; however  the authors overcame this to a certain extent by combining conditions into disease groups.

“The time-stratified case-crossover study aims to compare a case (or person’s) exposure immediate prior to the case-defining event (in this case hospital admission) with that same person’s exposure at other reference times.  This is an alternative to utilising time series analyses which has been used extensively in studies of the population level health impacts of air pollution. The method accounts for the personal factors which do not vary over time, and therefore overcomes the paucity of individual level data regarding other risk factors (such as genetic risk, health behaviour, lifestyle) and some of the challenges outlined previously.

“With regard to exposure assessment, the authors obtained PM2.5 measures US EPA monitoring data, which may be less reliable in rural areas with lower background concentrations.  Seasonal and climatic factors associated with both air pollutant and risk of hospital admission also merit further consideration; high pollutant episodes may also result in changes in medication regimens or access to care for those with chronic conditions, which in turn could increase hospitalisation risk. In addition, changes in the dominant PM2.5 sources are likely to have changed over the study timescale and further research is required to understand better the contribution of these, including the potential relationship with specific disease associations. It is also increasingly evident that indoor air pollutant exposure is a major factor for consideration, particularly among those who are less mobile.

“Provision of informal care (e.g. by family members) and associated opportunity costs has not been considered or costed within health economic analyses (presumably due to the limitations of data sources used) and therefore the overall figure is likely to be an underestimate.

“While the WHO is currently revising guidelines, this study adds to an emerging body of evidence concerning health risks of pollutant exposure below existing PM2.5 guideline levels. The relationships between risks posed by short and long-term (chronic exposure) and the sub-clinical or physiological outcomes are difficult to capture in existing epidemiological research approaches and require future investment in research to enable more accurate assessment. However, it is evident from the findings of this and many previous studies that reducing fine PM concentrations is essential to improving health and reducing healthcare costs among populations worldwide.”


‘Short term exposure to fine particulate matter and hospital admission risks and costs in the Medicare population: time stratified, case crossover study’ by Yaguang Wei et al. was published in The BMJ at 23:30 UK time on Wednesday 27th November.  

DOI: 10.1136/bmj.l6258


Declared interests

Dr Stefan Reis: Stefan Reis was not involved in this study, and declares no interests.

Dr Suzanne Bartington: I don’t have any other declarations – other than having reviewed the manuscript.

None others received.

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