Molds are fungi that grow in the form of multicellular filaments called hyphae that spread to form a network or colony called mycelium. There are thousands of known species of molds, although a much smaller number of mold species are commonly found in indoor environments. Molds can affect humans and animals in their vicinity in several ways. The most commonly known aspect of molds is the spores they produce as their primary means of reproduction. Spores are released from the mature mold body and spread by air currents and on people, animals, or materials that travel from place to place. These spores can remain viable for a long time until they find a suitable environment and grow to form new colonies. In addition to spores, mycotoxins can also be released under certain situations.

Mycotoxins are chemicals that are produced during certain parts of the mold life cycle and can evoke a “toxic” response (e.g., severe allergic reactions and respiratory irritation and exacerbation of asthma symptoms or other respiratory ailments) in humans and animals. Mycotoxins have low volatility, meaning they have relatively low concentrations in air, so contact or ingestion rather than inhalation is often the main route of exposure for these chemicals.

Mold can grow anywhere that satisfies four primary conditions:

  1. Presence of mold spores – spores are everywhere and it is very difficult if not impossible to remove them completely.
  2. Appropriate growth surface or nutrient source – molds are adaptable and can grow on almost any surface; many molds especially like cellulose-based materials (e.g., wood, drywall, insulation, cardboard, paper, carpet, etc.).
  3. Appropriate temperature – although many molds grow best in warmer temperatures, given enough time mold can grow at almost any temperature condition.
  4. Water – this is the most significant and most important criteria since the other conditions are too commonly available to be controlled.  The consensus of most organizations with a perspective on air quality (e.g., WHO, EPA, AIHA, ASHRAE, etc.) ͣ is that controlling moisture and dampness is the only way to consistently control or limit mold.

Mold Sources

Since there are so many possible locations that mold can grow, it can be difficult to locate without visual indicators.  However, there are some potential locations where molds are often found, as listed here.

  • Air conditioning units or drain lines
  • Near plumbing leaks
  • Near roof or wall leaks
  • Basement water intrusion from surrounding soil
  • Any consistently humid area
  • Near condensation around windows or any other condensation locations like exterior walls (typically where there  is a temperature gradient that allows water to condense)
  • Freezer/refrigerator door seals, especially in summer
  • Freezer/refrigerator drain line and drip pan (if present)
  • Indoor plants
  • Empty beverage containers and glasses, especially if left for trash or recycling without being rinsed out
  • Wastebaskets and trash cans containing discarded food or wet items
  • Sump pumps, especially when the pump does not cycle often
  • Stand pipes and traps
  • Books, magazines, and newspapers if they have gotten wet or sit for a long time
  • Outside mold, especially if the air intake is near the ground and landscaping near the home uses wood chips or mulch

Typically, if there is no plumbing leak, condensation, or water intrusion into the home, there will not be a mold problem. If active mold growth is indicated, the first step in fixing the problem is to find and repair the water intrusion or moisture build up.