Last month, two Toronto-area Realtors were arrested and accused of heading up a $35 million marijuana grow house operation. It's alleged that the Realtors leased client's dwellings for the purposes of setting up hydronic marijuana grow operations. An additional 37 people now face charges, mostly for cultivating and caring for the plants, but police say the two real estate professionals arrested were the leaders of the operation.

Growing marijuana in a home can be very profitable. Police estimate that each operation can produce about 1,600 plants a year and generate $1.6 million in profit.

Across Canada, police say there are more than 50,000 active grow houses. Most of them are in residential areas and the neighbours have no idea about what is happening next door. That's a big problem, says police, because grow houses are a serious danger to their communities.

The biggest danger is from a fire that could destroy the house as well as other homes close by. To set up the grow operations, a lot of electricity is required and operators usually tamper with the home's electrical supply to increase the amount of hydro being used, while bypassing the utility meters.

Police estimate that one in 10 of indoor marijuana operations will eventually go up in flames.

Electrocutions, caused by people rewiring the electrical system without knowing what they are doing, is another concern.

Police also say that because it costs operators $5,000 to $20,000 to set up a grow house, they often protect their investments by setting up "booby traps" to discourage intruders. These traps have included "live wires" on doors handles and windows, police say, which is a concern for emergency personnel who come to the house, or even innocent people such as newspaper carriers who may come in contact with the house.

Sometimes the furnace in grow houses is re-vented to circulate the air to feed the plants. This can result in backdrafting furnace fumes back into the house. There can also be a build up of poisonous gases from the chemicals used in the marijuana crop. These gases are also vented outside and released into the neighbourhood.

Another consideration is the cost of the stolen hydro -- estimated at $2,000 a month for each grow house -- and the property damage that is caused by renovations to the houses.

Last year, police across the country held several national campaigns that targeted marijuana grow house operators. Called Operations Greensweep, the blitz resulted in hundreds of arrests.

Utility companies, the insurance industry and the National Fire Marshal's Office were joined by The Canadian Real Estate Association last November to launch a public awareness campaign about the dangers of grow houses.

Here are some of the things that might indicate that a house in your neighbourhood is grow house: