Effects of humidity on the health of occupants of dwelling units: a literature review

The proposed research work sought to address indoor overheating in the 2025 edition of the National Building Code (NBC). The initial phase of the task aimed to address overheating by establishing an acceptable (evidence-based) upper limit value for indoor air temperature in dwelling units. The standing committee on housing and small buildings (SC-HSB) accepted the proposed requirement for an acceptable upper limit indoor air temperature of 26 °C. However, the committee agreed that the proposed change did not adequately address the overheating issue and that further research is needed to understand the interacting effects of indoor humidity and temperature on overheating risk in dwelling units. Study Objective Through multiple meetings with the Overheating Task Group, the research objective evolved to focus on the exploration of the health effects of indoor humidity in dwelling units at non-extreme indoor temperatures (around the proposed value of 26 °C). The objective of this literature review was to better understand the possible effects of indoor humidity at non-extreme warm temperatures around 26 °C on the health of occupants of dwelling units to provide evidence-based technical support for a proposed change to address overheating in the National Building Code of Canada (2025 edition). Method The literature was reviewed using a search strategy in the Scopus database and publications recommended through informal conversations with experts in the field of human adaptation and physiology. Certain reviews were also screened for relevant articles. Inclusion criteria were applied to the potential 3000+ articles. This comprised of having documented indoor humidity measurements, health outcome(s), and having new empirical data. Studies only looking at extreme temperatures above 30 °C were excluded. Health outcomes were divided into three categories; symptoms, morbidity, and mortality. Results The literature search returned 13 studies evaluating the effects of indoor humidity and temperature on human health. Nine studies focused on sleep (as one of heat-related health outcomes), two studies examined symptoms such as sick building syndrome (a condition where building occupants experience acute health/comfort effects linked to time spent in a building) and heat-related symptoms, and two studies characterized morbidity through distress calls for respiratory distress and diabetic crises. As expected, no studies with mortality outcomes met the inclusion criteria. Most studies found an effect of humidity on health (11/13) at varying degrees of temperature; however, the array of possible humidity thresholds cause the question of an indoor upper-limit of humidity to remain unanswered when looking at non-extreme temperatures. The heterogeneity in the results (i.e., wide range in temperature/humidity levels having negative health effects) may stem from methodological limitations or from the variability in study design and outcome measures. Overall, results suggest that: • sleep disturbances may occur as humidity and temperature increase, even below 26°C • respiratory distress may increase with rising humidity, especially over 26°C Conclusion Based on this review, the possibility of negative effects of humidity on health at varying temperatures (> 20 °C) is conceivable. With global warming leading to an increase in environmental water evaporation, resulting in higher humidity across the globe, considerations for the effects of humidity on health may become increasingly relevant. With sleep seemingly being a more vulnerable state for high temperature and humidity, implications REPORT No. A1-024303.1 PAGE 6 as to which indoor space should be thermally controlled and/or during which time period, may be important. It would also be important for ongoing and future research to generate additional data to evaluate the impact of indoor environmental conditions and other confounding parameters on critical heat related health problems. These critical health problems include but are not limited to cardiovascular disease, asthma, diabetes, respiratory diseases, and mental health issues. This review was unable to determine whether indoor humidity has an effect on building occupant health at non-extreme temperatures (< 30 °C). The available scientific evidence supports the proposed requirement for an acceptable upper limit indoor air temperature to sufficiently protect Canadian occupants in dwelling units against overheating risk without explicitly considering the interacting effects of indoor humidity and temperature.

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