Many health effects have been attributed to both long-term and short-term exposures to ambient PM (Table 4.1). Long-term exposure to ambient PM contributes to the initiation and progression of disease over months or years. Short-term exposure affects individuals who are particularly susceptible to the effects of PM; either because of existing chronic disease, compromised respiratory function in the developing lungs of children or, compromised physiological function in the elderly from the effects of ageing (Brook et al. 2010). Effect estimates (that is, the health effect per unit of PM exposure) in epidemiological studies are usually greater for long-term exposures than for short-term exposures, suggesting that long-term effects are not merely the sum of short-term effects (WHO 2013c). However, the health effects of long-term and short-term exposure are not independent. Repeated short-term exposures may result in the initiation and progression of chronic disease (WHO 2013c), while an acute event such as a heart attack or stroke that results from exposure during a day of high ambient PM concentration may be a consequence of chronic disease progression associated with long-term exposure.