DO NOT SLEEP UNDER A KOTATSU!
So far I have elaborated extensively on the plus sides of kotatsu, but there are also some risks that need to be considered when using, or before purchasing one. Especially when you have little children it is best to read these precautions carefully. Every year there are numerous examples of children who get seriously injured or worse because of improper usage of kotatsu. To avoid these horrible accidents we urge you to always be careful when using a kotatsu!
This is probably one of the most important rules when using a kotatsu. When you sleep you often move your limbs and some people also roll over. (I am sure you have the experience of falling asleep lying on your back only to wake up on your belly the next morning.) While this is completely normal unconscious movement under a kotatsu might result in you accidentally touching the heating element under the table, which can lead to serious burns! Also, depending on how deep you sleep you might dehydrate seriously while sleeping under a kotatsu table.
When you have small children it is important to not let them under the kotatsu, especially not when there is no supervision around! Toddlers and babies do not have the strength or intuition to get out from under a Kotatsu when it becomes too uncomfortable, and they do not have the proper communication skills to let you know. Pets and children can also get confused when under the futon of the kotatsu, which is usually pretty heave. This might make them feel like they are trapped under the kotatsu table, with no place to go.
While most kotatsus have an electrical heater placed under the low table, there are some heaters available that use fuel (usually kerosine) to warm the kotatsu. These devices have additional risk, coming from the fact that kerosine is is a highly combustible chemical. When dealing with kerosine (or any other highly flammable substance for that matter) proper safety precautions need to be taken. There are stories around of fires caused by kotatsus, not all of which are verifiable. When purchasing, if given the choice, I recommend would choose for the electrical kotatsu heaters if your situation allows for it.
When using electrical heating under your kotatsu table please consider the same safety precautions as with any electrical (heating) device!
Unfortunately there are some known cases in Japan where people (usually babies) died while under the kotatsu. I believe this can be avoided by always making sure there is adult supervision when using a kotatsu. While kotatsu tables are very comfortable devices which bring a lot of joy please be aware of the dangers that are inherent with using electrical or kerosine heaters!
On a cold winter night it is great to come home and dive into your kotatsu immediately. While this is fine, you should never leave your kotatsu turned on for this. If for some reasons a problem occurs with the heating element the consequences can be disastrous.