"I just moved into a rental house. Now all the former tenants boxes are gone I can see patches of chalky, white powder along my concrete basement walls. Is this a mould deposit? If so, what causes it? Is it dangerous or harmless and how do I get rid of it?"

There are several different kinds of white deposits that are often found on foundation walls. Fortunately, they fall into two simple categories. The first class of deposits is moulds. Some tenants and homeowners are horrified by the discovery of mould, even though there is often little reason to be alarmed. Some moulds present no health hazard whatsoever to humans. After all, moulds are a functional part of our eco-system and as old as life itself on the planet. However, some moulds do present health risks to humans.

The only sure way to determine the risk is to have a sample of the mould tested by a qualified laboratory. This type of work is the domain of an air quality investigator who will take samples of the mould as well as your indoor air and dust particles to determine if the moulds have spread to other locations throughout your home. Remediation is often a complex combination of factors that the air quality investigator prescribes after analyzing your building and site conditions to determine all the measures required to ensure the moulds do not return after the completion of a thorough clean-up regime.

The second possible type of deposit on your foundation walls is a harmless white powdery substance, a salty byproduct of a chemical reaction between water and the calcined limestone mixture in your concrete walls. The technical name for this powder is efflorescence. This white chalky substance indicates that there is seasonal dampness from outside your basement walls wicking through the concrete to the interior surfaces you observe. Efflorescence is a sign that you should be taking measures to control water in the yard adjacent to the affected walls. Often a simple set of gutters, downspouts and downspout leaders will significantly help dry the interior surfaces of these basement walls.