One of the very few really "toxic" molds can be easily recognized as that "black stuff" that grows on constantly-wet areas inside a building. However, it does require a constant drip to keep it alive, and fortunately, its spores are so heavy that they don't blow too far. This type of thing is easily spotted and remedied. There are, of course, hundreds of other types of molds, most of which aren't toxic - the type that grows on bread, cheese, etc., and various types of penicillin. Of course, many people are allergic to otherwise harmless molds, and for some these can be toxic.

One of the most common hidden places where mold grows is inside air conditioners. The environment is perfect for many types of molds, but fortunately, not for the most toxic (because of the cold location). It is dark and always wet on the evaporator (inside coil) and possibly wet inside the ductwork too. This is our most common indoor-air-quality problem in Florida. There we use a lot of attic-mounted horizontal electric furnaces that have a built-in evaporator coil. Since the inside of such furnaces is always damp, due to the close proximity to the cooling coil, they become prime breeding grounds for mold. And of course, the air distribution system delivers the spores throughout the building constantly, causing allergic reactions among some.

For mold to survive it needs two things-Soil and water. Soil inside an air conditioner is created when dirt is allowed to accumulate on the coil and other interior surfaces (e.g. fan blades). Dirty filters and cheap throw-away types promote this. Probably the worst single offender is the insulation inside the furnaces and ducts (e.g. fiberglass). Its coarse surface collects dirt and debris. Just add water (from the wet evaporator coil or from a clogged and overflowing condensate drain), and voila, mold.

To alleviate this problem, many people hire duct-cleaning companies. The problem with this though, is:

1. Duct cleaners usually don't clean the source of the problem, the indoor coil and air conditioner.
2. Duct cleaning only cleans surface mold and dirt, the spores remain just to grow again.
3. Duct cleaning loosens the fiberglass insulation fibers, causing another source of irritation in the air.
4. A moisture problem is still there.

Recommendations

First, use high-quality air filters and keep them clean.

Second, clean the inside (the wet area) of the air conditioner with a vacuum cleaner once a year to minimize standing "soil."

Third, Make sure that the condensate drain (the water from the indoor coil) is kept draining by pouring a little bleach down it once a year. The most common reason for clogs is mold growth that fills the pipe.

Fourth, have an air-conditioner technician clean the indoor coil once per year with a non-corrosive (soap-based, non-acid, non-alkali) cleaner.

Fifth, especially with horizontal furnaces (generally in attics), hire a technician to make sure that the air velocity isn't high enough through the indoor coil to cause water to blow off into the ductwork. All he or she has to do is feel the ductwork downstream from the evaporator to see if the insulation is wet after running the system for a few minutes.

Two after-market devices that can help to alleviate an air-conditioner mold problem are ultraviolet lights and electronic air filters. Ultraviolet lighting that shines constantly on a wet area will completely eliminate mold growth. However, the bulb must be changed annually to remain effective. Electronic air cleaners do an excellent job of cleaning the air entering the air conditioner, without restricting the airflow. However, they may crack and pop (since a high voltage is used) and they have to be cleaned regularly. I don't recommend electrostatic (non-powered) air cleaners or cheap, thin (1 inch thick) types since they do very little when it comes to removing mold spores. Remember that "electrostatic" types require low humidity to develop a significant charge, and that isn't the case in an air-conditioned building where the relative humidity usually doesn't drop below 50% (65% in Florida).

Of course, keeping mold from growing inside air conditioners can be greatly reduced by design. Starting with the ducts, the finest choice for all reasons is externally-lined sheet metal. This type accumulates little "soil" for mold to grow in due to its smooth surface. And the zinc coating on the steel is a natural anti-microbial (antiseptic).

However, if the ducts must be internally lined, my first choice would be insulation with a smooth foil liner facing the airflow side. There are, however, specially-treated fiberglass liners, but the surfaces should be as smooth as possible to prevent accumulation of dirt.

Flex-type ducts resist the growth of mold since they usually have a smooth inner plastic liner, but their aerodynamic properties are usually very poor, so their use should be limited to short runs between the main trunk and diffuser grills.

On the equipment side, an air-conditioner coil mounted above a vertical furnace is the best choice, because the vertical design doesn't allow a place for standing water. However, the insulation in the indoor-coil area should at least be foil lined. Where horizontal furnaces are used, or where the equipment is a "package" type (the whole unit is outdoors), specify top-of-the-line models with smooth, covered, internal insulation. Also, make sure that the condensate drain lines are large enough to discourage clogging.

So, while not being really technical, this is the letter I sent to my friend. Perhaps you have customers and friends who are concerned about this same issue. Why not mail a copy of this article or post it where it can be read.