In every community, there are individuals whose chances of illness or accident are greater than those of others. In susceptible persons, adverse health effects occur at a lower exposure than in the majority of population. The reason for this, though often weak, can usually be ascertained. The paper focuses primarily on host factors that may increase an individual's susceptibility to air pollutants present in the general environment. These factors include genetic background, age, gender, nutritional status, physiological status, presence of coexisting lung disease, and lifestyle. They relate to various mechanisms of individual susceptibility to air pollutants from the environment into the body, to alterations in detoxification and immunological reactions, and to variations in the responsiveness of lung tissues. Several epidemiological techniques to detect biological events relevant to host susceptibility and disease progression are mentioned. They are mainly based on genetic traits, host characteristics, pulmonary function tests, biochemical and immune changes. Measuring the variations in risk among susceptible individuals is necessary to assess correctly respiratory health risk due to inhaled pollutants at the population level, and subsequently to develop rational public health policy to reduce the incidence of those diseases in a given population. Before being able to formulate this policy, however, one must know how to detect those individuals who are susceptible to air pollutants.