A number of conditions besides the old saw "accessibility" or size of the crawl space opening should be considered carefully by the home inspector, electrical inspector, or anyone who is deciding if it is safe to enter an crawl area anywhere in a building. 
Here is a list of some safety and health considerations that the inspector should evaluate before deciding to enter a crawl space:

• Is there standing water in the crawl area? If so there is risk of electrical shock (if wiring or electrical devices are present). There may also be a chemical contamination risk, especially in older buildings where pesticides may have been applied in the crawl area.
  
  
• Is there excessive debris in the crawl area? Nails, splinters, and possibly rodents may be in the debris in our photo at left.
  
  
• Are there chemical odors in the crawl space? If so there is an increased risk of chemical contaminants that could be hazardous. You should not enter such an area without proper protective clothing, respirator, etc.
  
  
• Is there evidence of asbestos insulation, especially disturbed,damaged, or deteriorated asbestos insulation? Do not enter such an area without protective equipment; take care that you do not track hazardous materials out of the crawlspace and into other building areas. Often we find a crawl area in which the asbestos pipe insulation is not just hanging (photo at left) but has fallen onto the crawl space floor.
  
  
• Is there evidence of mold contamination such as areas of wood, paper, or other material covered with mold or mold-suspect material. Do not enter such an area without protective gear.
  
  
• Is there evidence of rodents or snakes or insect pests in the crawl space? Rodent hazards include bacterial and viral and respiratory illness; there is the obvious risk of snake bites in a confined space, and more than once we've been run out of a crawl space by bees or hornets. But since you're unlikely to be able to move rapidly to make an emergency retreat from threatening pests, crawl areas are riskier than some other building areas. Evidence of pests may also suggest risk of improperly applied and unsafe exposure to pesticides.