The level of humidity is important to all chelonians. Where that humidity is (AKA air or substrate humidity) can also be important. Without proper humidity, dehydration is a major problem. It can also cause eyes to seal shut or get puffy and it can cause some tortoises to have very watery eyes or bubbly noses. Too much humidity can cause pneumonia and respiratory problems, especially in cold environments. It can also cause shell rot in aquatic turtles. Each species have their specific preference regarding the level of humidity, so, again, I urge you to also read the specific care sheets.
For aquatic turtles, low humidity is not at all an issue as they live in water. However, high humidity can be a concern because it prevents the shell from drying off completely when the turtle is basking itself. Many of the microorganisms that lead to shell rot need constant moisture, so drying off completely kills them and prevents problems. This is especially true when the turtle is shedding its scutes because water will get trapped under the loose scutes and it takes longer to dry out. For this reason, it is advisable to never have a solid cover on your aquatic turtle's tank but rather to either leave the top open or to use a screen top. This allows evaporating water from the tank to escape, thus keeping the tank's humidity level nearly the same as the ambient humidity level of the room.
If it is very cold, I have sometimes found it necessary to use a hood to trap in heat. However, when doing this it must be noted that heat lamps cause water to evaporate more quickly and if you see a sort of "fog" forming under the hood, the humidity is probably too high. I always leave a section of the hood open and I remove it entirely everyday while feeding or otherwise maintaining the turtles, thus allowing extra moisture to escape out.
For all other turtles and tortoises, humidity is provided in the substrate. Most times I find it sufficient to provide a hide box or two with moist substrate in it. (see my substrates page.) However, if your turtle or tortoise is having humidity problems (dry skin, closed or puffy or watery eyes, clear bubbly nose, reluctance to feed except perhaps on the most moist foods, and frequent soaking/drinking), it is a good idea to cover half to three-fourths of the enclosure with the moist substrate, especially under the basking lamps. The heat from basking lamps is incredibly drying, so this area is where you would want to provide the most humidity. Make sure you turn over the substrate at least once a week as oftentimes the top layer will appear moist but in fact most of the underlying substrate is very dry and dusty.
If this does not seem to help, making use of a humidifier in the room for at least part of the day or adding mister to the enclosure may help by raising air humidity. Tortoises especially get their humidity from substrate and not from the air, but a higher air humidity will help to keep the substrate from drying out as quickly.
If too high, though, it can cause respiratory problems, so this only comes second to watering down the substrate more thoroughly. It's also important to use the appropriate substrate. For example, long-fiber sphagnum moss holds much more moisture than a 50/50 mix of Eco-Earth and play sand, and therefore it is the only thing suitable for hatchling redfoot tortoises. It holds too much moisture for a desert tortoise, however.
Also make sure you are soaking your turtle or tortoise an appropriate number of times a week. Usually adults should be soaked twice a week, or three times if they are sick or dry. Young ones may be better off having a soak every other day. However, double check with your pet's specific care sheet because too frequent soaks will add too much water to a tortoise's system and cause food to move through quickly without being digested.
Humidity control is a small part of chelonian keeping. I personally have never used a humidity gauge as it is simple to tell how wet the substrate is simply by testing with a finger. The animal's behavior can tell you the rest. Despite this, it's still an important thing to watch out for.
