Canada needs to develop a remediation standard for dealing with houses that have been used as marijuana grow operations, says a new report from Canada Mortgage and Housing Corp. (CMHC).

An increasing number of grow ops are being discovered across the country, leaving mortgage lenders and sometimes unsuspecting new homeowners with dangerously contaminated homes.

"The cultivation of large amounts of marijuana in confined spaces gives rise to safety issues involving mold from excess moisture, as well as contamination from the use of fungicides and insecticides, various solvents and other chemicals used for various purposes," says the CMHC report by project managers Virginia Salares and Marie Dyck. "Walls, ceilings and floors can be contaminated throughout the house and especially in the grow areas. Growers may have disposed of excess chemicals in an unsafe manner such as down the drains inside or dumping outside the house."

CMHC sampled 12 former grow ops across the country, testing the homes for indoor air quality including moisture and mold problems. All but one of the homes had been raided by police and all plants, chemicals and growing equipment had been removed from the empty homes. One house was being lived in by a person who bought the home without knowing it had been used as a grow home, and only found out later about the home's history. The previous owner had tried to hide damage to the home by painting over mold-covered walls and ceilings.

"CMHC has since been contacted by several home buyers in similar positions," says the report. "Although it is not known how often this occurs, it has been related by law enforcement officers that it is becoming more common for marijuana growers to buy and sell the (grow ops) within a short time. The rapid recycling of houses is intended to avoid detection by neighbours and police."

The homes are then sold to unsuspecting buyers who are exposed to hidden contaminants.

Many municipalities now publish the addresses of homes that have been used as grow ops. In Ontario, the government says it considering launching a provincial registry of former grow ops.

Some municipalities in B.C. and Alberta have also passed bylaws that set out requirements for the remediation of grow ops, but the CMHC report says these requirements vary from municipality to municipality. "In the absence of applicable bylaws, limited guidance is available to property owners responsible for (grow op) remediation," says the report. It says a nationwide harmonization of standards should be developed.

The 12 houses in the study were thoroughly investigated and a report to the property owner was prepared. Most of the houses needed "extensive and costly" repairs, says the report. In one, where a garage was used as a grow op for an extended period, demolition of the garage was recommended.

In most of the homes, holes had been cut in walls and ceilings to vent moisture from the grow operation into the attic, the garage, or up a chimney. Some of the homes had disconnected heating ducts, and some had additional wiring and electrical assemblies. Electrical panels had been tampered with.

"In all of the subject houses, the investigators stressed the need to gut the basements and all other rooms used for growing, and to ensure that there was no mold in the insulation or wall cavity after the drywall was removed," says the report. "Contractors specifically trained in mold remediation were recommended for this purpose."

The report notes that "typically, training for mold remediation contractors is for mold cleanup and does not include identification of the causes and corrective measures. The investigation is a necessary and independent step prior to the remediation." CMHC recommends that although training programs specific to marijuana grow ops are not yet available in Canada, investigators should include qualified structural, electrical and HVAC contractors, as well as indoor air quality investigators.

"More work is required to ensure that training for indoor air quality investigations of (grow ops) is appropriate and available," says the report. Since former grow ops may be vacant for a long time after a raid by police, the report says the homes are often subject to severe water damage caused by burst pipes. This happens when the utilities are disconnected during winter. The report says "it is strongly recommended that if the utilities are to be disconnected, the water supply be turned off by police after the investigation is concluded and the grow op dismantled. The homeowner or property manager should ensure that the system is drained."

CMHC is continuing its research about remediation procedures that must be developed to deal with dangerous chemicals used in grow ops.