Some health effects may show up shortly after a single exposure or repeated exposures to a pollutant. 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. Soon after exposure to some indoor air pollutants, symptoms of some diseases such as asthma may show up, be aggravated or worsened.

The likelihood of immediate reactions to indoor air pollutants depends on several factors including age and preexisting medical conditions. In some cases, whether a person reacts to a pollutant depends on individual sensitivity, which varies tremendously from person to person. Some people can become sensitized to biological or chemical pollutants after repeated or high level exposures.

Certain immediate effects are similar to those from colds or other viral diseases, so it is often difficult to determine if the symptoms are a result of exposure to indoor air pollution. For this reason, it is important to pay attention to the time and place symptoms occur. If the symptoms fade or go away when a person is away from the area, for example, an effort should be made to identify indoor air sources that may be possible causes. Some effects may be made worse by an inadequate supply of outdoor air coming indoors or from the heating, cooling or humidity conditions prevalent indoors.

Other health effects may show up either years after exposure has occurred or only after long or repeated periods of exposure. These effects, which include some respiratory diseases, heart disease and cancer, can be severely debilitating or fatal. It is prudent to try to improve the indoor air quality in your home even if symptoms are not noticeable.

While pollutants commonly found in indoor air can cause many harmful effects, there is considerable uncertainty about what concentrations or periods of exposure are necessary to produce specific health problems. People also react very differently to exposure to indoor air pollutants. Further research is needed to better understand which health effects occur after exposure to the average pollutant concentrations found in homes and which occurs from the higher concentrations that occur for short periods of time.

Bacteria and viruses that cause respiratory infections thrive in extremely high and extremely low humidity.
Mold spores, dust mites and other allergens survive best in high humidity environments.
Higher humidity can increase the levels of noxious chemicals in the air, which may include ozone and formaldehyde.

Poor indoor air quality caused by improper humidity levels can cause a wide range of problems, from making your family sick to damaging your walls, floors, and furniture.

High humidity is more of a problem in the summer, and is often found in homes with inadequate ventilation and poorly functioning air conditioning systems. High relative humidity conditions (over about 50-60%, depending on the temperature) can cause problems such as:
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Damp air resulting in mold growth that can cause musty orders as well as symptoms ranging from nasal stuffiness, eye irritation, wheezing, or skin irritation to dangerous lung infections.
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High concentrations of dust mites can aggravate asthma symptoms.
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High humidity can make warm temperatures feel even warmer, leading to an increase in heat-related illnesses like heat rash, muscle cramps, heat exhaustion, and even heat stroke.Damp air creates condensation which damages indoor building materials.

The majority of adverse health effects caused by relative humidity can be minimized by maintaining indoor levels between 40 and 60%. The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) recommends temperature ranges of 68°F to 78°F at 50% relative humidity as comfortable for most homes.