Dr. Sarah Janssen, a physician and scientist for the Natural Resources Defense Council, an environmental advocacy group, says that she frequently sees patients complaining of symptoms related to toxic levels of mold, lead, formaldehyde and tobacco smoke.
The sources of these toxins and poisons are often easy to isolate: a leaking roof can cause mold, houses built before 1978 may have lead paint and plywood can off-gas formaldehyde.