Generally considered acceptable, but some pollutants may affect a small number of individuals who are unusually sensitive.
People with pre-existing conditions like asthma, lung disease, or heart disease, as well as children and the elderly, may experience symptoms.
Everyone may begin to experience some adverse health effects, with sensitive groups being more affected.
Health alert level; the entire population is likely to experience more serious health effects.
Emergency health warnings; everyone may experience more severe effects.