When it comes to UV there are more opinions than can even be covered in one article. The sad reality of it is, like most other things when it comes to iguanas, even the well meaning decently educated people don’t have a true understanding of UV basics. This, more than anything, is due to the number of opinions that are available out there. Up until recently, I thought I **knew** UV, then I purchased a book or two and found that I didn’t really know anything at all and what I did know was so basic that it didn’t even begin to scratch the surface.

In this article we will cover UV in both basic and not so basic terms. While you will find some information pertaining to artificial UV lighting, both fluorescent and non-fluorescent, the main point of this article is not to discuss the pros and cons of the specific brands of bulbs (that will come in another article).

In the wild iguanas utilize the UVB of natural sunlight to produce D3. D3 is essential for calcium absorption. No matter how much calcium an iguana is getting whether it is through the food they eat or vitamin supplementation, if there is a lack of D3, the calcium is not doing your iguana any good. Why is D3 and calcium so important to an iguana? Because without the D3 an iguana can’t absorb the calcium which plays a supporting role in normal bone formation.  A lack of calcium can and will lead to metabolic bone disease otherwise known as MBD. MBD is covered elsewhere on this site.

One of the most frequently asked questions we come across is how much UV does an iguana need to be healthy? To the best of our knowledge, to date, there has been no scientific research done which can answer this question. Even Hatfield of “Green Iguana: The Ultimate Owner’s Manual” fame, sidesteps this question in his book. I find it rather disturbing though that he and Melissa Kaplan, after both admitting that in the wild iguanas spend the better part of their days in full sunlight, claim that all one has to do is allow for one half hour of exposure to natural, unfiltered sunlight a day for their pet. It just doesn’t make any sense to me at all. If the best way to ensure health in captivity is to mimic an iguana’s natural habitat, then one hour of sun a day isn’t going to do much at all. Furthermore, since NO commercially produced UV light, whether fluorescent or non-fluorescent, produces the same amount of UV that the sun does, how can we be assured that they are going to provide an iguana with enough UV for D3 production?  

So, how much? I bet you’re still asking!

The best explanation of how much UV to give I can offer to you comes from David Krughoff. Both on his website at http://www.myiguana.com and in his books “Anna My Green Friend” and “Reptile Lighting 2000”, he breaks down the often complicated discussion of UV into terms that the rest of us can understand. This is a good place to interject that I have corresponded and spoken to David on several occasions and while I may not agree with everything he says, I have learned a great deal from him and value him as one of my teachers immensely. I also want to point out that I can say the same of Kaplan and Hatfield (i.e. –I don’t agree with everything they teach but I have learned some valuable information from them) although I have not corresponded with either of them the way I have with David.  The following paragraph is all based on information from the three Krughoff sources I mentioned above.

Interesting fact –“The international standards for measuring UVB are microwatts per centimeter squared - or milliwatts per centimeter squared - or watts per meter squared.” D. Krughoff http://www.myiguana.com

David Krughoff did some testing with his handheld radiometer back on June 29, 2000, which was a clear sunny day. In Hoyleton, Illinois, he spent the day taking readings of the sun starting at 7 am and repeating the readings every hour on the hour until 7 pm. The highest UVB output from the sun that day was 269 microwatts/cm2 and that reading was taken in full sun at 1:00. At 1:00 he also went into the shade and took a reading of 54 microwatts/cm2. We know that iguanas are diurnal creatures so we need to look at the readings during daytime hours as well.
7am     12
8am      74
9am     142
10am    192
11am    233
12 noon 156
1pm  269
2pm  262
3pm  239
4pm  187
5pm  131
6pm  61
Source “Reptile Lighting 2000” David Krughoff

For those of you interested, you can view the rest of his readings at his website or in his book, but for our purposes here today we only need to look at some of the readings. Now we all know that Hoyleton, Illinois is not the tropics by any means! Can you imagine how much higher the readings must be in the tropics!


If, in the wild, an iguana is spending most of its day out in the sun and during the hours between 9am and 5pm we can deduce from David’s readings that UVB output is at its highest, how can one half hour a day of UV be enough for an iguana? In Hatfield’s book, he recommends a 24 hour period that you maintain a photoperiod of about  12-14 hours of “light” and the rest darkness. So if we understand him correctly, of the 12-14 hours of light, only 30 minutes of that needs to be UV exposure. That’s can’t be right. Can it? Not when in nature an iguana is getting at least 7 hours of strong UV exposure.

How much UV? In terms of hours, it is our belief, based on the research we have read, that an iguana should be getting a minimum of 8 hours of UV exposure per day but 12 hours would be better. In terms of actual UVB, we should be offering the closest we can to what the sun itself would be providing.

How can we be sure that isn’t too much UV and that our iguanas won’t produce too much D3 and die?

“ Also photo biologists agree that D3 production is self limiting. Otherwise we would have a lot of dead prairie farmers, road construction workers and wild green iguanas!

Those interested in learning more about vitamin D3 will enjoy http://www.photobiology.com If you do a search of vitamin D3 on this site; the results will keep you busy for awhile! Anyway there are excellent articles about the self limiting component of D3 production on this site. This self limiting factor is observed even when 7 dehydrocholesterol is exposed to UVB radiation in ampoules. After 3 % of the dehydrochlesterol is isomerized to previtamin D3 the process becomes self limiting.”
Source: David Krughoff http://www.myiguana.com



So, how do we provide enough UV? What are the options available to those of us who can’t provide year round exposure to natural sunlight?  We look to artificial sources of UV. Please read our article about commercial UV sources.
UV -What's The Big Deal?
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