Probable Health Risks Due to Exposure to Outdoor PM2.5 in India

Abstract:

Particulate matter of size <2.5 µm (commonly referred to as PM_2.5) is considered to be the best indicator of health risks due to exposure to particulate pollution. Unlike the decreasing trends in the developed countries, aerosol loading continues to increase over the Indian subcontinent in the recent past, exposing ~1.6 billion population at risk. Lack of direct measurements prompted us to utilize satellite data in establishing a robust long-term database of surface PM_2.5 at high spatial resolution. The hybrid approach utilizes a chemical transport model to constrain the relation between columnar aerosol optical depth (AOD) and surface PM_2.5 and establish mean monthly conversion factor. Satellite-derived daily AODs for the period 2000-2012 are then converted to PM_2.5 using the conversion factors. The dataset (after validation against coincident in-situ measurements and bias-correction) was used to carry out the exposure assessment. 51% of the population is exposed to PM_2.5 concentration exceeding WHO air quality interim target-3 threshold (35 µg m^-3). The health impacts are categorized in terms of four diseases - cardio ortho-pulmonary disease (COPD), stroke, ischemic heart disease (IHD) and lung cancer (LC). In absence of any region-specific cohort study, published studies are consulted to estimate risk. The risks relative to the background concentration of 10 µg m^-3 are estimated by logarithmic fitting of the individual cohort studies against the corresponding PM_2.5 concentration. This approach considers multiple (>100) cohort studies across a wide variety of adult population from various socio-economic backgrounds. Therefore, the calculated risks are considered to be better estimates in relative to any one particular type of risk function model (e.g. linear 50 or linear 70 or exponential). The risk values are used to calculate the additional mortality due to exposure to PM_2.5 in each of the administrative districts in India to identify the vulnerable regions. 52.1% of the additional mortality is attributed to COPD, while the corresponding numbers for stroke, IHD and LC are 19.3%, 25.2% and 3.4% respectively. In absolute terms, an additional 0.6 million (with an uncertainty of ±20%) mortality per year is estimated in India due to exposure to outdoor PM_2.5 in the last decade.