The likelihood of immediate reactions to indoor air pollutants depends on several factors. Age and existing medical conditions are two important factors. In other cases, whether a person reacts to a pollutant depends on individual sensitivity, which varies from person to person. Some people can become sensitised to biological pollutants after repeated exposures, and some people can become sensitised to chemical pollutants as well. 

Immediate effects may show up after a single exposure or repeated exposures. These include irritation of the eyes, nose, and throat, headaches, dizziness, and fatigue. Such immediate effects are usually short-term and treatable. Sometimes the treatment is simply eliminating the person's exposure to the source of the pollution if it can be identified. Symptoms of some diseases, including asthma, hypersensitivity pneumonitis, and humidifier fever, may also show up soon after exposure to some indoor air pollutants.