Dear James: There always seems to be a musty/mildew odor in my house, which is quite old. Could the odor come from the part over the crawl space? What can I do to improve the air quality? —Mike Y. Dear Mike: Constant moisture and dampness are typically the most common causes of musty odors and mildew. If your house is damp, the nasty odor may be the least of your problems. Mold spore from the mildew can cause allergic reactions, and some types can be toxic. It can be inviting to termites and ants, and it could also damage the supporting lumber for your house. Moisture in a house comes from both indoor and outdoor sources. It can be generated internally from bathing, cooking, or even a humidifier set at an unreasonably high level. It can also come from outdoors through a leaky foundation, slab, or roof. High indoor humidity …