The Intergovernmental Panel on Climate Change (IPCC) indicates that human activities have increased the moisture content or absolute humidity of the atmosphere . The absolute humidity is the mass of water vapor divided by the mass of air. The relative humidity, which is more often reported, indicates the amount of water in air relative to the maximum amount of water vapor air can hold at the air temperature. The projected effects of climate change on absolute humidity are complex and variable. Regional increases and decreases in both precipitation and temperature, and changes in airflow patterns caused by climate change will also cause associated regional increases and decreases in absolute humidity.
Overall, the indoor air absolute humidity is likely to rise and fall with the change in local outdoor air absolute humidity. However, changes in buildings will likely modify this relationship. In particular where use of air conditioning increases substantially as a result of climate change, the time average indoor air absolute humidity may diminish because air conditioners remove water vapor. The impact of increased air conditioning on indoor absolute humidity will depend on air conditioner design and operational factors. Thus, one can only state that there will be local increases and decreases in indoor air absolute humidity as a result of climate change.
The indoor air humidity affects the survival and number of house dust mites, although the relationship varies with the type of dust mite. These mites are an important source of allergens that contribute to allergic and asthmatic symptoms, and to the development of asthma [56, 57]. The level of exposures to these allergens early in the life affect whether one becomes sensitized to (allergic to) dust mite allergen. The available data link indoor relative humidity to dust mites, although insufficient research has been performed to untangle the effects of relative humidity versus absolute humidity. When relative humidity is maintained below the 40% to 50% range for a prolonged period, dust mites die [58, 59]. This is the case for many homes where the winter is cold and dry, causing a low relative humidity in the heated indoor air . By increasing average winter outdoor temperatures, climate change could theoretically, in some areas, enable dust mites to survive indoors throughout more of the year, increasing allergies and asthma at least during the winter. When the indoor humidity is sufficient to support dust mite reproduction, mite levels increase substantially as the humidity increases [59-62]. Thus, in these situations changes in indoor humidity caused by climate change and associated changes in use of air conditioning could lead to corresponding changes in dust mite levels which may in-turn affect allergies and asthma.
The overall net effect of climate change on allergies and asthma due to house dust mites is uncertain. More information on dust mites, their relationship with indoor humidity, and their health risks is available in the section of this web site addressing indoor dampness. A review of the potential effect of climate change on dust mite levels and associated allergies and asthma is provided by the Institute of Medicine .