Building Biology Logo
Building Biology


Since 1993

What About Mold?

Three conditions must be met to have mold:

  1. A food source
  2. Fungal spores
  3. Moisture

Food for mold is everywhere- plants, plant debris, wood, fabric, paint, paper- anything with carbon and hydrogen. Mold spores are part of the dust that settles on everything. Because of this, the key growth condition for getting mold is moisture. Some molds can grow when the air moisture or humidity near their place of residence reaches 70%. Other molds require up to 90% local humidity and this takes actual liquid water. This liquid water can be provided by:

  1. Condensation
  2. Water Leaks
  3. Rising Damp

The percentage relative humidity or RH is the ratio of the amount of moisture in the air to the amount of moisture the air can hold at its current temperature. If the air temperature decreases the relative humidity increases. Water vapor from the air condenses on a surface, the surface temperature has decreased to the dew point temperature (DP). The DP temperature is the temperature at which the relative humidity of the air reaches 100% and no more water vapor can be held by the air. When the air temperature decreases any further water droplets appear on the surface. Also note that the DP temperature is a function of the air moisture content. The more moisture the air holds, the lower the DP temperature This is what happens on the inside of a windowpane when itís cold outside. This why we see dew on the grass some mornings. The heavier the dew, the lower the surface temperature of the blades of grass.
Sources of moisture in indoor air are:

  1. Human Activity
  2. Leaks
  3. Rising Damp
  4. High Outside Humidity

Leaking Water
Leaks can be from outside water entering through structural defects such as roof problems, flashing defects and siding gaps. Flashing is used around windows and doors and where the roof meets vertical walls. The purpose of flashing is to guide outdoor water harmlessly away from the structure. Leaks can be from the inside of the house from waste water lines, pressurized water supply lines or from water consuming appliances. The most insidious leaks are slow leaks that wet materials inside walls, but are not active enough to be seen inside the house.

Rising Damp
Rising damp describes water movement through porous materials via capillary action. Capillary action occurs when the space the water passes through is small enough that the normal surface tension of water propels the water through the space even vertically against the force of gravity. The tendency of water to move via capillary action can be seen by looking closely at the surface of water in a glass. Where the water meets the glass, the water is beginning to creep up the side of the glass forming an area called the meniscus, the curved upper surface of a column of liquid.

The concrete slab upon which many homes are built is the most common source of rising damp. Slabs should be poured on material that provides a capillary break like crushed rock. When that is not done correctly, ground water moves up through the soil into the concrete slab and up into the house. This increases air moisture content and increases the temperature at which condensation occurs.

Even in situations where the capillary break is present, excessive watering, poor grading and lack of rain gutters can so wet the earth near the edges of the slab that rising damp still becomes a problem around the outside perimeter of the house. Floor areas with rising damp have elevated moisture levels in the air which increases the dew point temperature to a point that condensation is a frequent occurrence. Moldy carpeting in perimeter areas of a slab on grade house indicates a rising damp problem.

Mold Spores are Everywhere
Even if your house has no mold growth, indoor air will contain mold spores because indoor air comes from outside. Outside mold grows naturally and releases spores. The levels and types of fungi inside a home depends on whatís outdoors. If a home has moisture problems causing fungal growth, the indoor quantity and species of fungi will be different than the outside and will show fungi found on water-damaged materials. In order to make this comparison air samples are taken inside and outside in order to provide enough information to help confirm or deny active inside mold growth.

Testing for Presence of Mold
Testing to determine if active mold growth is occurring in a building can be done in numerous way.
The most prevalent methods are:

  1. Air samples which are evaluated by a microbiology lab as to fungus identity and quantity.
  2. Surface samples which are evaluated by a lab as to identity, number and sometimes growth fragments.
  3. Dust samples that are evaluated as to fungus identity and quantity.
  4. Air samples which are evaluated for microbial volatile organic compounds (MVOC).
  5. Dust samples that are evaluated for the presence of mycotoxins.

Tests to identify MVOCs and mycotoxins are expensive and seldom used. Today testing for mold contamination is usually limited to collection and identification of spores as in 1, 2 and 3 above. Air sample and surface sample are the most popular methods. Because of cost considerations even in these more common methods, identification of mold is only to the genus level (a genus contains many species) even though conclusions might be more meaningful if all the species were known.

Testing Cost and Handling Built-in Variation in Sampling
Everything in our world has built in random variation. The supply of mold spores varies with air current, time of day and even season. The collection rate of sampling equipment has variation. The lab that studies the sample has slight variations in measurement techniques. The science of statistics deals with the estimating whatís occurring in the real world despite the variations in whatís being measured.

What does this mean so far as determining if an inside space is free of active mold growth? It means that more than one sample must be taken at each location. Being conscious of cost to the client, yet desiring somewhat reliable data, the minimum number of samples is three per location- one location inside and one location outside. With this minimum number of samples the level of spores inside has to be at least five times the level outside to state with some assurance that there is active inside mold growth. As more samples are taken per location, the multiplier drops below five, but the cost to the client increases.

Testing is Secondary
Sampling for mold is often talked about, but is really a secondary consideration. Often testing is not even needed to determine is mold growth is a problem and where the moisture is coming from that is causing the problem. Building history, client observations and expert inspection and measurement of moisture in materials should be the combined if needed with sampling and lab testing to determine if there is active mold growth in the building. In this field experience counts for a lot. Anyone can collect samples, but few have the experience and persistence needed to see what needs to be seen and integrate all the information gathered to draw an informed conclusion.

Toxic Mold
"Toxic mold" is a meaningless term, used by the poorly informed. It is probable that virtually all mold spores have the potential for causing allergic response depending on the individual and the level of exposure. Additionally, a limited number of molds make mycotoxins. Most mycotoxins are poisonous, however, some like Penicillin and Cyclosporin are medically beneficial. Mycotoxin is not always produced and production occurs under poorly understood conditions. Sensitivities to mycotoxins probably vary widely.

Individual Response Varies
Individual allergic response to mold spores has been scientifically demonstrated. Individual trigger points are variable. The amount of an allergen needed to elicit a serious response in a sensitive individual could be thousands of times lower than that needed to invoke so much as a sniffle in an equally healthy, but non-sensitized person.

Exposure Standards
Because of the differences in individual susceptibility, no mold exposure standards exist. There is little understanding of why people in an obviously mold infested house suffer health problem, whereas, most people outdoors in relatively humid climates with very high spore counts do not have similar health problems.

There is controversy over the role mold plays in indoor problems. Because of a lack of research, evidence against mold is primarily based on observation. Unfortunately, for some people years of scientific observation is not proof enough. It seem reasonable, but it is not proven that indoor health problems are probably caused by the accumulation of spores, microbial volatile organic compounds (MVOC) and sometime mycotoxins.

The Best Defense is a Good Offense
Since spores are everywhere, the best defense against mold growth is a dry house. Find and stop causes of condensation, leaks and rising damp. Dry out wet materials within 36 hours before spores sprout and rapid mold growth begins. Discard anything that canít be dried out in this time. This usually means that wet drywall, carpet pad and upholstered furniture must go. Do this and you only have to deal with a water problem, not a mold problem.

Who to rely on?
It is best to get an expert, independent inspector to determine if there is a mold problem, the extent of the problem, and how the problem should be remediated. The person doing this should not be connected to the company that will do the remediation. Since there are many over night wonders in the mold business, look for length of experience and recognized certification. Check references. Worthwhile certifications are from:

  1. Indoor Air Quality Association
  2. American Indoor Air Quality Council
  3. Institute of Inspection, Cleaning and Restoration Certification

Top of Page

Telephone: (805) 644-2008 |