See the Heating, Lighting and Humidity section for specific information on the proper use of UV bulbs in your iguana's enclosure. Iguanas require high humidity as well as high temperatures in their environments. One of the most common problems seen in captive iguanas is dehydration. Iguanas don't seem to be programmed to drink very much, perhaps because in their native rainforests there is sufficient humidity in the air and moisture in their food to keep them hydrated. Captive environments tend to be much drier, and with a lack of desire to drink, even if water is made available, many iguanas spend much of their time at least mildly dehydrated.
Chronic dehydration taxes the kidneys, and can contribute to kidney failure at fairly young ages.
For this reason, it is important to provide your iguana with a humid environment. High humidity also helps loosen shedding skin, which makes the shedding process easier and helps prevent retained shed. For ideas on how to provide humidity, see the Heating, Lighting and Humidity section. As you can see, providing your iguana with the appropriate habitat conditions is quite a challenge. However, it is important to realize that it is necessary to do so.