Mold and Mildew – Emerging Public Health Concern

Jan 28, 1999

Outbreaks of mold and mildew in buildings appear to be on the upswing, sometimes resulting in foul odors, allergic or toxic reactions of occupants and obvious aesthetic concerns. Although fungi have always been here, they are only recently emerging as a public health and indoor air quality concern.Mold, mildew and mushroom-like growths are all types of fungi that rapidly reproduce when conditions of moisture, appropriate temperature and food (anything organic containing carbon) exist. The visible portions of fungi are the fruit, and they produce spores, the seeds of the organisms, which are everywhere in the environment at low levels. However, indoor infestations can result in highly elevated levels of airborne spores. People inhale fungus spores constantly, and healthy individuals are seldom affected. But some spores carry toxins that can result in sickness or allergic reactions in some people.

The most notorious is a mold called stachybotrys charturum, which is suspected in several deaths since its first victim was reported in 1986.The common denominator for mold and mildew growth is excessive indoor moisture. Today’s energy conservation codes require increasingly airtight buildings, particularly homes. When the amount of water vapor created within a structure exceeds the capacity to dissipate it through ventilation or other means, conditions ripe for fungal growth exist.Sources of water vapor include defective or insufficiently dry concrete slabs on grade, cooking, bathing, uncovered aquariums, houseplants, and human breathing. Sometimes, excess moisture also results from leaking plumbing or from rain leaks in exterior walls, windows, doors or roofs. Multi-family housing appears to be particularly susceptible to mold and mildew outbreaks because many projects are built on concrete slabs and have fewer exterior walls and attics than conventional homes. Re-circulating range hoods and low volumes of makeup air for HVAC systems can also exacerbate conditions otherwise conducive to fungal growth.

Experts generally agree that indoor humidity should be kept below 50 per cent. In an adequately ventilated and well-constructed new apartment in the San Francisco Bay Area, winter indoor relative humidity should be expected to fluctuate in the 35 per cent to 50 per cent range. Relative humidity measurements taken in several projects with elevated mold growth typically range in the 50 per cent to 80 per cent level.

Diagnosing Moisture Sources

Relative Humidity can be measured by simple digital hygrometers, but recording hygrothermgraphs that provide week-long charts provide inexpensive methods for spotting trends in temperature and humidity. These can be rented for about $150 per week from national suppliers.Water vapor emissions through or from concrete slabs can be measured using a variety of methods, the most reliable of which is the anhydrous calcium chloride method. Several methods, including the Calcium chloride method are described in ASTM E 1907-97 Standard Practice for Determining the Moisture-Related Acceptability of Concrete Floors to Receive Moisture-Sensitive Finishes and F 1869-98 Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride. Calcium Chloride tests cost about $150 per location.Fungi samples can be identified and quantified by a microbiological laboratory with mycological expertise. One such service is Forensic Analytical with laboratories in San Francisco and Los Angeles. The web site is

Preventing Indoor Moisture Buildup

Some apartment building managers and condominium associations are being proactive in warning residents about lifestyle-related activities and conditions that promote mold. A list of considerations includes:

  • Respiration and perspiration of occupants. Each person expels an average of about 0.67 quart of water into the air in a full 24-hour day. Some of the units with the highest number of individual occupants generally have the highest rates of fungal growth.
  • Bathing or showering. A five-minute shower will add about 0.13 quarts of water to the air in 24 hours. Wet towels or clothes left to dry will add additional water vapor.
  • Uncovered aquariums.
  • House plants. Five to seven average plants will add about 0.25 quarts of water in 24 hours.

Although not contributing to the amount of water in the air, an occupant can exacerbate already high humidity by:

  • Not operating the HVAC system. Some systems have a fan that can be operated alone at a very low electricity cost.
  • Not running the bathroom exhaust fan during and after bathing and showering
  • Not opening windows, particularly during the day.
  • Storing large volumes of personal possessions such as clothing and boxes in closets, under beds, and in other constricted spaces in a manner that inhibits air circulation.
  • Leaving soiled clothing around or not cleaning furnishings and carpets.
  • Frequent cooking, especially in a manner that results in boiling large amounts of water.

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