Homes used as growhouses

Source: Institut national de santé publique du Québec Bulletin d’information en santé environnementale (BISE)


In the last several years, the culture of Cannabis sp, also known as marijuana, has seriously expanded across Canada. Because the climate in Québec is cold for a good part of the year, offenders tend to favour indoor over outdoor growing. More productive and more discreet, these installations allow residential growers to harvest three or four crops a year, versus just one outdoors. Thus between 1981 and 2004, the number of marijuana growing offences has increased by 460%. In 2004, the types of operations were broken down as follows: 47% inside of dwellings, 34% in wooded areas and 19% on agricultural land. Based on statistics compiled by the various Québec police forces, since 2000 there have been several thousands of marijuana growhouses in Québec (see diagram on the next page). A high proportion of these houses are found in the area comprised of Montréal, Laval, Lanaudière and the Laurentians.

A lucrative crop

A mature cannabis plant is worth between $1,000 and $1,500 on the resale market, making this a very lucrative crop. In eight years, the percentage of the psychoactive substance contained in the cannabis produced in Québec, called tetrahydrocannabinol (THC), has gone from 5.5% to 11.5%, making Québec ''pot'' a very desirable product. In addition, it is relatively easy to grow between 200 and 900 plants (300 to 500 on average) in a well-equipped basement over a period of only four months.

Conditions conducive to fungus contamination

Cannabis can be grown indoors using a series of containers of potting soil irrigated by automatic systems; it can also be produced using more sophisticated hydroponic or areoponic systems. In a closed hydroponic system the roots are in permanent contact with a complete nutrient solution, whereas in an aeroponic system, they are in contact with air and a mist of nutrient solution coming through diffusers. The presence of these systems indoors creates relative humidity rates between 40% and 90% (usually around 85%). In addition, cannabis growing requires powerful lighting (up to sixty 1000-watt lamps) and temperatures between 25°C and 28°C. These hot and humid conditions are optimal for marijuana growing, but also for the proliferation of moulds. Moulds are live organisms that grow naturally outdoors. They can be brought inside by air movements and the comings and goings of humans and animals. In order to develop and multiply, they require the presence of appropriate environmental conditions (humidity and temperature) and nutrients such as those found in building materials (wood and paper cellulose). As we have seen, all these conditions are rife in dwellings used as growhouses.

Impacts of fungus contamination on a building and the health of its occupants

Apart from the risk of explosion due to solvents used in making marijuana oil, or fires caused by electrical malfunctions, a high rate of humidity maintained over long periods is the main cause of damage to a building. High humidity rates cause building materials to rot and moulds to proliferate. Widespread fungus contamination is associated with potential health problems for the occupants of the dwelling. These can be very severe and go from respiratory tract ailments or asthma exacerbation to even more serious symptoms for those more at risk. People who have predisposing factors such as a chronic respiratory disease or a weak immune system can develop mycotoxicoses, hypersensitivity pneumonitis, and even respiratory or systemic infections. Decontamination of a mould-infested home is sometimes possible, albeit costly. According to the Insurance Bureau of Canada, effective decontamination of a major fungus infestation costs an average of $40,000. Even at that, in some worst-case scenarios, after two or three crops little can be done to save an infested house, and razing becomes the only solution. Given this, it is easy to see why offenders will first try to mask the damage. In some cases, damaged walls are repainted or even replaced. This cosmetic work can temporarily hide problems caused by chronic humidity, but is usually not enough to eliminate the mould, as the humidity accumulated in the structures frequently resurfaces in materials, whether they are new or simply fixed up to look good.

Clues that a property has been used as a growhouse

Many types of properties are used for marijuana growing (cottages, apartments, suburban homes, etc.) and they are spread out throughout the province. Inside the dwelling, several clues that a high humidity rate was maintained over a long period of time can tip off the fact that a property has been cosmetically altered for resale purposes. They include the relative humidity rate, odours (sometimes hidden by perfumes), the presence of visible moulds, bubbling of gyproc walls, blackened woodwork or round stains on the floor. But the presence of these signs is often not enough to sow doubt in the mind of an inexperienced buyer who visits the property, and so the problems can be difficult to detect before purchasing. These concealed defects can even stay hidden until the new occupants undertake renovation work requiring the opening of walls. This is where the sometimes disastrous extent of the problem is brought to light. Other revealing clues are sometimes visible from the outside, such as barricaded widows, the presence of several ventilators on the roof, a modified or very recent electrical mast, wiring that goes in directly through a window (without going through the electric meter), or a Hydro-Québec meter with a broken seal. In winter, ice forming outside the kitchen and bathroom fans as well as excessive condensation and frost on windows are telling signs of an abnormally high humidity rate. Finally, the presence of condensation and especially of moulds on cold surfaces, such as the north wall, are signs to watch for.

The legal environment

In addition to financial losses (property devaluation, cost of decontamination), growhouses resold in poor condition to unsuspecting buyers can pose serious health risks for the new occupants. But the legal environment can make it difficult to prevent these fraudulent sales. The legal aspects related to marijuana growing include the notions of offence-related property and proceeds of crime. A growhouse is considered an offence-related property since it was used to commit an illegal act. In addition, the offender’s vehicle and his bank account can also constitute proceeds of crime if these are found to have been acquired with profits resulting from crime, a fact that is often difficult to prove. This nuance becomes important since the growhouse used to commit the crime becomes seizable if the offender is the owner. However, it is not always possible to identify the culprits or even the owners, or indeed to intervene quickly enough. Following a search in a growhouse, the police can lay charges and request a freeze order, legally preventing a person from disposing of the building concerned or from carrying out a transaction involving it. Although the order is always served to the accused, the owner of the immovable, the municipality and the mortgage creditors, the time lapse – even if short – is sometimes enough to allow the offenders to make cosmetic alterations to the house and resell it, sometimes with the help of an ill-intentioned real estate agent. The freeze order is also published by the Bureau de la publicité des droits, but only a week or two after it has been issued, again allowing time for illicit transactions. Currently, the Sûreté du Québec is advising buyers or public health authorities who want information concerning a house to go to the courthouse of the region concerned.

Creation of a register

The delays involved in publishing cases and the multiple sources of information make potential buyers and the real estate agents representing them vulnerable to fraudulent transactions. The Association des courtiers et agents immobiliers du Québec (ACAIQ) deems this situation unacceptable and is demanding that its members have access to a common register that would list all the properties for which a search warrant has been issued. Such a tool would enable a real estate broker or agent to inform potential buyers in real time. The Sûreté du Québec is currently reviewing the possibility of setting up an information exchange protocol between the professionals concerned.

Benefits of partnerships

In addition to the legal difficulties surrounding this issue, many stakeholders are concerned with the health problems associated with the presence of moulds in these homes and would like to develop partnerships in order to find an effective modus operandi to identify unhealthy dwellings and prevent the sale or lease thereof. To date, two activities have helped educate stakeholders about the benefits of partnerships.

Seminar organized by the Association des courtiers et agents immobiliers du Québec (ACAIQ) (2)

In addition to regularly informing its members, in fall 2005 the ACAIQ held a seminar on the different aspects of the growhouse problem. On this occasion, the Association invited representatives from Health and Social Services, Public Security and Justice, police forces and building inspectors, financial institutions, insurance companies and real estate brokers and agents. The two main objectives were uniting to better fight this problem and educating the public. The event was a first in Canada. The seminar received major media coverage, thereby helping to inform a vast public about what many have come to consider a veritable plague.

Meeting with the Sûreté du Québec (SQ)

In 1999 the Sûreté du Québec set up a program to fight the expanding production and traffic of marijuana. The main purpose of this program, called Opération Cisaille (3,4) is to destabilize the criminal organizations that are overseeing marijuana production and distribution. Although the efforts of this special squad have borne fruit and upwards of 700,000 plants were seized in fields in 2005, police recognize the importance of partnerships, including with public health authorities, in dealing with the increasing number of growhouses and the threat they pose to the health of future occupants. According to the SQ, homes searched during the operation that were used as grow operations for two to five-year periods were completely contaminated by mould. Aware of the social issues involved and the harmful effects on health, the Sûreté du Québec has deemed it relevant to alert the Table nationale de concertation en santé environnementale (TNCSE), which includes an environmental health representative from each of the health and social service regions, to the magnitude of this phenomenon in Québec. Because these operations pose a health risk for many people, police are seeking the help of public health authorities in an effort to find a workable solution to limit the resale and lease of contaminated dwellings. Potential solutions are currently being examined.


In the last few years, indoor marijuana growing has considerably expanded in Québec. The environmental conditions created in growhouses (high temperatures and humidity rates, presence of organic matter) are conducive to the proliferation of mould. The extent of fungus contamination observed in these environments is such that it poses a health risk for potential occupants. However, the current legal environment makes it difficult to prevent fraudulent transactions. The solution could involve creating a register of homes for which search warrants were issued, as well as the development of partnerships. In this regard, the collaboration of public health authorities could prove beneficial in resolving the problem.