Some molds are harmful, others are benign. And, how mold affects you can depend on your own personal sensitivity. Mold expert Jeff May learned this first hand. The Johns Hopkins University Press author of three books (My House is Killing Me, My Office is Killing Me and the Mold Survival Guide), wasn’t always such an accomplished expert on how mold can make you really sick.

“For years, I had an office air conditioner that was probably too big for the space. As a result, my office as always damp and I would cough whenever the AC came on. Then one day I opened it up and found that everything inside had turned black with Cladosporium mold. That was the defining moment when I put two and two together,” said Jeff May.

According to May, the first step to avoiding mold is to understand what makes it tick. Mold needs three things to grow: moisture, air and food, and this combination can be found just about anywhere in your home. “Mold grows where it can find food: the dust on a bathroom ceiling, the starch paste on the back of wallpaper, or the plant fibers that make up the jute pad under a carpet. Add moisture, and mold growth begins,” said May.

Here’s what you need to know to keep your home mold-free:

• MIND THE MOISTURE Keep humidity below 50% in basements. Improve outside grading and drainage by keeping gutters clean and soil always sloping away from your home. Cover dirt crawlspace floors with plastic to reduce moisture.

• STORE SAFELY Keep all storage at least several inches up off concrete floors and away from foundations where dampness can easily seep in. This is especially important with organic material like cardboard boxes. Avoid using wooden shelves; metal or plastic shelves are preferable.

• HEAT FINISHED BASEMENTS Below grade spaces like finished basements are more likely to become infested and should always be heated to at least 60 degrees, even when not being used. The warmer the space, the less the chance that condensation will form and feed a mold problem.

• BUILD MOLD RESISTANT When choosing building materials, use materials that won’t feed the mold. Tom Combs took this option when remodeling the bathroom in his family’s 1990 lake house outside of Atlanta, Georgia. “The ceiling was covered with mold and I wanted to take immediate action before the situation worsened,” said Combs. His solution was the Dens Armor Plus, a wallboard made by Georgia Pacific that is specifically designed to prevent mold growth. Unlike regular drywall that has a paper face, Dens Armor Plus has a fiberglass face that cannot feed a mold problem.

• VENTILATE VIGOROUSLY Poor or missing ventilation fans in damp spaces like baths and kitchens can leave enough moisture behind to sustain a mold problem. Make sure all baths and kitchens are vented by properly sized fans that take moisture outside and NOT into attics. Keep the bathroom door open after bathing to speed drying of surfaces.

• AVOID BASEMENT CARPETS More than almost any other material in a house, carpets can be incredibly effective havens for mold. Even non-organic carpets can collect dirt, dust and moisture that combine to provide mold a fertile ground in which to grow, especially in below-grade spaces where relative humidity tends to be higher. Hard surface products like laminate flooring or engineered hardwoods are always a better choice for basement spaces.

• FILTER THE AIR If your home has a forced-air heating and cooling system, using a top quality air filter is a must. May recommends pleated filters with a MERV rating of at least 6-8, or 11 if the family is prone to allergies. Another option is a whole house electronic air cleaner. Mounted permanently to the home’s HVAC system, a whole house air cleaner uses ionization technology to charge particles making them stick to filters like a magnet. According to Consumer Reports, the most effective unit is the Aprilaire Model 5000, which can trap virus-sized particles as small as one micron (one millionth of a meter) and needs just yearly filter replacement.

• INSULATE DUCTS Duct systems that carry heated or cooled air throughout your house must be insulated whenever they pass through unheated or uncooled spaces like attics or basements. If not, condensation can form inside the ducts and, when combined with dust in the air, can allow mold to grow in the ducts, and then spores can easily circulate throughout your entire house.

• CLEAN CAREFULLY Use mold-inhibiting cleaners in bathrooms and kitchens. Portable air conditioning units should be taken apart and cleaned at the start of every season. When painting damp spaces like kitchens and bathrooms, use paint with a mold inhibitor EPA-approved for indoor use.

• FIX FLOODS FAST f you do have a major leak or flood, quick action can stop mold before it starts. Thoroughly dry soaked carpets and padding, and remove any wet upholstery. Then wash and disinfect all surfaces before the carpet and pad are replaced.

You can also do mold testing to help narrow down the cause of the mold. Hiring someone who understands buildings before you do a mold test, such as home inspector who is trained in mold and a member of the American Society of Home Inspectors (ASHI), can help you find the problem and change the conditions, so the mold won’t return.