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  • Writer's pictureMillines Herman

Why does Relative Humidity Hurt

Relative Humidity (RH) is the amount of moisture within the air as water vapour. It is measured and expressed as percentage needed to reach the saturation point, at the same temperature. Therefore, if the air contains one quarter of the amount of water it can hold, it is said that the relative humidity is at 25%. RH. At 100% is the point of saturation, where the air is unable to hold water. The EPA’s Building Air Quality Guide posited that if air contains all the water it can hold, it is at 100% RH, and the water vapor condenses, changing from a gas to a liquid (United States Environmental Protection Agency, 1991).

Relative humidity is directly related to temperature. Temperature affects relative humidity levels, as it determines the saturation point (dew point). The lower the temperature, the lower the ability for air to hold water. In assessing an indoor environment, it is always important to understand the current relative humidity levels, temperature levels, dew point and also surface temperature levels. If the surface temperature is lower than the environment’s temperature, condensation or the risk of condensation is likely because of the chances of RH levels reaching its dew point.

A literature review of health effects of humidity written by Baughman and Arens, noted ‘The primary influences of humidity on health are through pollutants.’ (Baughman & Arens, 1996). They also listed some of the health issues commonly associated with biological pollutants.

Infectious disease (pathogens)

Bacteria (e.g. Streptoccucus, Legionella)

Viruses (e.g. common cold, flu)

Fungi (e.g. Aspergillus Fumigatus)

Allergic reactions (e.g. Asthma, rhinitis)

Dust mites (dried body parts and fecal excreta)


Nonallergic immunologic reactions (e.g. hypersensitivity pneumonitis)




Fungi” (Baughman & Arens, 1996)

All of these are possible with RH levels at and above 60%. A research paper entitled ‘Indirect Health Effects of Relative Humidity in Indoor Environments’ coined that maintaining RH levels between 40% and 60% would minimize majority of adverse health effects caused by relative humidity (Aurendel, Sterling, Biggin, & Sterling, 1986). This notion is also supported by the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) in its ASHRAE Handbook 2017 (ASHRAE, 2017).

H&L Environmental Services Ltd have collected data over that last few years of the ambient environment in Saint Lucia and relative humidity levels have been ranging from 65% - 90%. Therefore, our environment readily supports pollutant buildup, but we as employers, home and building owners have the responsibility to ensure that our indoor environment is kept safe.


  • ASHRAE. (2017). 2017 ASHRAE Fundamentals. In ASHRAE, 2017 ASHRAE Fundamentals I.P Edition (p. June). Retrieved November 12, 2019

  • Aurendel, A. V., Sterling, E. M., Biggin, J. H., & Sterling, T. D. (1986, March). Indirect Health Effects of Relative Humidity in Indoor Environments. Environ Health Perspect., 65, 351-361. Retrieved November 12, 2019, from PMC US National LIbrary of Medicine:

  • Baughman, A. V., & Arens, A. E. (1996, January). Indoor Humidity and Human Health - Part 1: Literature Review of Health Effects of Humdity - Influenced Indoor Pollutants. Retrieved November 11, 2019, from

  • Canyons and Crags. (N.D.). Relative Humidity. Retrieved November 12, 2019, from

  • United States Envrionmental Protection Agency. (1991, December). Building Air Quality Guide: A Guide for Building Owners and Facility Managers. Retrieved November 10, 2019, from EPA:

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