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lung and elsewhere, because they are more likely to have developed chronic cardiorespiratory diseases, or because their place of residence and modes of travel expose them to higher levels of urban pollution. 5.2 Impacts of vulnerability The concept of vulnerability indicates that increases in exposure to pollution may have substantial effects on a vulnerable portion of the population, even if the change in risk for the whole population is small; and, conversely, that reductions in pollution levels may lead to pronounced health benefits in population groups with the highest vulnerability. Individuals, or groups of similar individuals, who are especially vulnerable to the effects of air pollution may: • for any given exposure to pollution, have a higher risk of an adverse health outcome (Fig 15a) • for any given exposure to pollution, experience a more severe adverse health outcome (Fig 15b) • experience an adverse health outcome at a lower level ('threshold') of exposure to pollution (Fig 15c) • be more likely to experience an above-threshold exposure to pollution (Fig 15d). The adverse effects of air pollution measured in epidemiological studies are depicted as an average of effects across a whole population of individuals, within which there will exist varying individual states of Fig 15. Different impacts of air pollution vulnerability. Vulnerable population Risk of adverse effect a b d Pollution dose Risk of adverse effect Pollution dose Severity of adverse effect Pollution dose Norm Likelihood of exposure Pollution dose Vulnerable population c Norm Vulnerable population Norm Vulnerable population Norm © Royal College of Physicians 2016 69 5 Our vulnerable groups