Homeowners can use checklists to narrow down the possible sources before testing, unless the problem is thought to be an emergency. For example, suspected carbon monoxide should be tested for immediately. Some pollutants produce an odor that allows the homeowner to detect them. Natural and LP gas are examples. The smell of gas should be a signal to contact the gas company immediately for assistance.
Sewer gas is another example.
Some pollutants have no smell, pose serious health threats and are common enough that all homeowners should consider testing for them. Everything that burns produces some carbon monoxide. Households with combustion appliances should be sure these appliances are regularly checked by professionals for safe operation. A draft-hood test for combustion air is one test the heating professional should do. This test can determine whether appliances vent properly or are releasing combustion gases into the house.