I know hydrogen is highly flammable.
If you really understand what you are doing and have enough money to purchase good equipment you can probably be safe. However doing something like a fuel cell on a small budget is probably asking for a dangerous and problematic situation.
You should start very small and never allow it to ignite in a sealed container. If the pressure becomes too much, you will see lots of warning signs. As long as you are not reckless, you will be fine. If you start to build one, you should keep in mind that not all distilled water is really distilled water; some times it is only deionized water. Also, you will be miles ahead if you use sodium hydroxide (lye) instead of common table salt. A pressure release valve should be installed and tested before you start work on the timing, and a diode should be installed well before that. It might seem unnecessary at that stage but it will save money in the long run.
