Second hand tobacco smoke
Second hand tobacco smoke is bad for everyone, including pets. Avian veterinarians say that tobacco smoke can cause chronic eye problems, skin irritation and respiratory disease, such as coughing, sneezing and sinusitis. When a bird stands on a hand of a smoker, nicotine from the smoker's fingers can be absorbed by the skin of the bird's feet, which can lead to contact dermatitis. Some veterinarians believe that tobacco smoke may be a contributing factor in feather plucking. Please do not smoke around your birds, or let others smoke around your birds. If you do smoke, wash your hands thoroughly before handling your birds.
Some laser printers generate something akin to second hand smoke and could be dangerous for birds that are kept near the printer. For details, see the article.
Incense and candles
Mold and fungus
See also the article on Aspergillosis.