Final answer:
The most common problem with using wood as a major heat source in the United States and Canada is air pollution caused by the combustion of wood.
Step-by-step explanation:
In the United States and Canada, the most common problem with using wood as a major heat source is air pollution caused by the combustion of wood. When wood is burned, it releases harmful pollutants such as carbon monoxide and particulate matter. These pollutants can have negative effects on both human health and the environment.
To mitigate this problem, it is important to use modern wood stoves or fireplace inserts that are designed to release smaller amounts of particulates and maximize efficiency. Additionally, sustainable logging practices and proper forest management can help ensure that the wood supply is replenished and minimize the impact on ecosystems.