Crawlspaces are host to a large number of conditions that may harm the house. Never enter a crawlspace without proper personal protective equipment.
Crawlspaces are notorious for the nasty discoveries because their cool, dark environment attracts undesirable pests and can promote dangerous conditions. Since crawlspaces are mostly unmonitored, hazards can breed there unchecked for long periods of time.
More common dangers discovered in crawlspaces are:
Dirt crawlspaces provide the environment that is enjoyed by ants, termites, and various other pests. Termites cannot survive long outside of their mud tubes, which you may see on foundation walls and piers. Carpenter ants should be plainly obvious as well, and both of theses pests can cause structural damage. Also bear in mind that where there are pests, there may also be pesticides, perhaps improperly applied.
Standing water or sewage
Dirt crawlspaces are susceptible to water seepage, which can create a host of problems, such microbial growth, odors, damage to stored belongings, and risk of electrical shock.
Mold and fungus
Just like pests, mold and fungus can grow rapidly in crawlspaces. They are both a health concern as well as a cause of wood decay, which can require a costly repair. Airborne mold spores can potentially enter the living space from the crawlspace. Molds produce allergens, irritants and, in some cases, potentially toxic substances. Inhaling or touching mold or mold spores may cause allergic reactions in sensitive individuals. Homes infected with molds and fungus are also much more difficult to sell, often requiring costly remediation prior to closing the deal.
Do not disturb asbestos! The microscopic fibers that cause illness become airborne when the insulation is handled or disturbed, and if it appears to be in good shape, it might not be a problem at all. Prolonged exposure to asbestos insulation can cause mesothelioma, which is a cancer of the lining of the chest and the abdominal cavity, as well as asbestosis, in which the lungs become scarred with fibrous tissue.