Standing water and sewage are big health risks for anyone entering an under-home crawlspace. Mold and fungi are also often found.
Perhaps something many people are worried about with an under-home crawlspace, and for good reason, are insects, rodents or small animals. Some of these could be poisonous or dangerous if they feel cornered or threatened. Even if you want to make a hasty retreat, this will be challenging in such a tight space.
Structural issues with the home are another reason why an under-house crawlspace can be dangerous. The best thing to do if you suspect the home has these types of issues, is to have a professional look at the property first. However, if it is completely necessary for you to enter such a space, its very important that someone is informed of your whereabouts and can call for help should you encounter a problem. Its also important that you are equipped with the appropriate protective clothing and gear so that you can be as safe as possible.