All of the above-mentioned conditions that are unpleasant to humans can be the ideal place for mold and fungi to grow. Standing water or sewage could also be found in an under-home crawlspace. These are all problems that come with known health risks.
Rodents, insects or other animals are another issue you might encounter. There is a real need for caution with this as some creatures might be poisonous or dangerous, especially if they start to feel threatened or cornered and it would also be very difficult to move out of the way in such a tight space.
If you are concerned that the home may have some structural problems it is important that you do not enter the crawlspace. Instead, have a professional come and perform an inspection on the home to make sure you don’t put yourself at risk.
If you decide that it is necessary for you to enter the under crawlspace of the home, make sure you wear the proper protective equipment and clothing so that you avoid being exposed to whatever may be down there. It is also extremely important that someone else knows you are down there and what you are doing so that they are available to raise an alarm if you run into problems.