It is for this reason that you cannot just set your iguana in front of a closed window in the sun. An additional source of UV light is special fluorescent UV bulbs available in pet stores that sell reptile supplies. Some people feel that if daily doses of real, unfiltered sunlight can be obtained on most days, then the use of artificial UV light bulbs in the iguana's enclosure is not necessary. The effectiveness of real sunlight to stimulate iguanas to produce vitamin D3 varies with the time of year and latitude of your location. 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.