




































Iguana 10.3 color covers (Page C1)


R
eaders will recognize that, in order to
stay healthy, iguanas require UV-B
(ultraviolet radiation in the “B” range)
as much as they need the complex salads

we prepare for them daily. Without UV-B, the all-
important mineral, calcium, cannot be effectively
absorbed — and calcium is as critical as nutritious
food, water, and heat. In responsible husbandry,
we must replicate the UV spectrum required for
the photochemical process involved in metaboliz-
ing vitamin D3 (which mediates calcium absorp-
tion) — while never forgetting that this will be of
little importance if the diet, heat, etc. are less than
optimal. Also remember that this is a discussion on
what is best for reptiles, NOT humans.

Since extensive research published in reputable
scientific journals has yet to be performed, the rec-
ommendations that follow are based on logic,

atomic physics, engineering principles, existing
knowledge about the natural habitat of iguanas
and their biology, and my own personal experi-
ence.

UV-B is part of the electromagnetic spectrum.
This spectrum includes everything from radio
waves at one end to gamma rays on the other.
Visible light is somewhere in the middle.
Wavelengths are read in nanometers from 0.001
nm (x-rays) to 100 billion nm (radio waves). A
reptile bulb can be subjected to testing by an ultra-
violet radiometer and spectrographs in order to
measure precisely the wavelengths that are being
produced.

The UV range is from 180–400 nm, the B
range specifically from 280–320 nm. However, we
are most concerned with readings from 290–300
nm. Why? This is the D-UV range, which triggers
the miracle of photo-biosynthesis and is responsi-
ble for creating pre-vitamin D3 (cholecalciferol).
We think that about 80% of this photochemical
reaction is triggered by ultraviolet waves in this
range. Ingested vitamin D (7-dehydrocholesterol
or 7-DHC) in the reptile’s skin absorbs the UV-B
protons, which allows the photochemical reaction
that converts the 7-DHC to cholecalciferol. The
latter is then converted in the liver to 25-hydrox-
ycholecalciferol (25-HDCC). The final step
toward becoming biologically active vitamin D3
(1,25- dihydroxycholecalciferol or 1,25 DHCC)
takes place in the kidneys through the process of
thermal isomerization. Biologically active vitamin
D3 is stored in the liver and kidneys and its pri-
mary function is to regulate calcium metabolism.
This gives you some idea of why reptiles suffering
from metabolic bone disease (the lack of sufficient
D3 to metabolize calcium) also suffer from forms
of liver and kidney disease.

Having mentioned reptilian skin, let’s quickly
consider the amount of exposure of an animal to

H U S B A N D R Y
82 IGUANA Volume 10, Number 3

Iguanas and Artificial Ultraviolet Light: 
How and How Much Made Simple — 

Well, Not Exactly Simple…1

Bob MacCargar
The Greenhouse in New York

All photographs by the author.

Proper placement of bulbs will provide an iguana access to
heat and UV light and permit proper thermoregulation by
allowing the animal to move toward or away from the heat
source.

1 All references to brand names reflect the author’s
personal experiences and should not be construed as an
endorsement by the IIS.



natural sunlight that is necessary for this chemical
reaction to take place. The assumption that a few
minutes a day is sufficient is based entirely on stud-
ies with humans. Keep in mind that the protons
from the UV radiation have to penetrate the skin
deep enough to reach the capillaries underneath
the skin in order to produce this reaction. Anyone
with experience around iguanas can appreciate
how tough and thick their hide is. Several hours a
day of natural sun exposure is much more appro-
priate for a large reptile.

However, the ultraviolet we need to supply for
our animals must be “useable” (in other words, in
the D-UV range). How can we measure and be
confident that we have supplied our creatures with
ample amounts of D-UV? By far the most reliable
method is to test specifically for blood levels of 25-
HDCC (wild iguanas have levels of 175–275
nmol/L of this form of D3 in their blood). This
test can be performed by a qualified veterinarian
and is the final say on whether we have provided
optimum husbandry for our iguanas. This is much
more reliable than using a standard blood panel
test (which measures only calcium and phospho-
rus levels). Even iguanas that have what appear to
be good blood panels have been found to be on
the low end of the active 25-HDCC level. This
helps to explain why iguanas in the wild can drop
20 feet out of a tree and hit the ground running,
whereas stories of captive Green Iguanas falling 5
feet and ending up with compound fractures are
common.

Just what kind of ultraviolet levels are these
creatures exposed to in their natural environment?
The standard for measuring intensity of the UV
spectrum is read in microwatts per square cen-
timeter (µW/cm2). The USDA recently did a
study with an Ultraviolet Pyranometer and found
readings of UV-B in Florida on June 1st that
reached 450 µW/cm2 (remember, however, that
only a certain percentage of this falls into our
“usable” D-UV). Hobbyists have the ability to
take their own UV-B readings with a simple hand-
held ultraviolet radiometer from Solarmeter
(model 6.2). The measurements taken by the
USDA are extremely close to the readings that I
have been taking for the last two years in a study
of reptile lamps and I have used this as a constant
for meter calibration.

Although iguanas may not be exposed to as
much as 450 µW/cm2 on a continuous basis, they

will spend several hours a day exposed to relatively
high numbers. My studies, as well as those of oth-
ers, have found that, even in the shade, global UV-
B readings reach 30–50 µW/cm2. These numbers
give us an idea of the minimum and maximum
ultraviolet B exposure levels in nature.

Interestingly, another study has proven that
D3 biosynthesis is a naturally self-limiting process.
Without getting too technical, this “safety valve”
ensures that toxic levels of vitamin D3 are not cre-
ated, and that the excess is broken back down into
inert ingredients (but please see the references at
the end of this article). Basically this means that as
long as we do not expose our iguanas to any more
UV than that to which they are exposed in their
natural environment, we will stay within safe
perimeters.

Two styles of reptile UV-B bulbs are available.
One is the fluorescent tube and the other is the
mercury vapor (MV) reptile lamp. Both style
lamps use the heavy metal mercury as a catalyst for
producing ultraviolet radiation. An electrical
charge passing through liquid mercury excites the
molecules until they vaporize (when the mercury
cools, it resumes liquid form). In the fluorescent
tube, the mercury must combine with high-grade
phosphorus to achieve the ultraviolet results.

Over the past two years, I have studied failure
and decay rates of mercury vapor lamps and con-
ducted a general study of the major brand fluores-
cent tube reptile bulbs. I am constantly asked if I
have tested a certain fluorescent brand bulb by
individuals who have seen them listed inexpen-
sively — in spite of the fact that, for many years, I
have been telling people that quality UV is not
cheap (unless we’re talking about the great and
wondrous sun). In a recent conversation with

83IGUANASeptember 2003

The low-cost hand-held
ultraviolet radiometer from
Solarmeter (model 6.2)
provides hobbyists with the
ability to take their own 
UV-B readings.



Voltarc Technologies, one of the largest manufac-
turers of reptile fluorescent bulbs in the nation, the
engineers confirmed the fact that money buys UV.

Production costs rise with more exacting spec-
ifications (how much UV-B and where precisely it
is to be delivered). In order to build a tube that
will provide not only high UV-B readings, but one
that will generate output in the useable 290–300
nm range, manufacturing tolerances have to be
very precise. Two different fluorescent tubes can
emit equal amounts of total UV-B, yet one will do
a much better job keeping your pet healthy than
the other (even though both bulbs might have
been manufactured by the same company, but for
two different distributors’ specifications).

The best fluorescent tubes tested emit 12–15
µW/cm2 at 12" after initial burning. A variety of
good fluorescents (as well as some absolutely ter-
rible ones) are on the market, but ZooMed 5.0 is
built to the most exacting tolerances according to
all of the manufacturers with whom I have spoken. 

The other choice in artificial UV is the mer-
cury vapor reptile lamp, which comes in a variety
of styles and wattages. Anyone who has done reha-
bilitation work has seen the effects of these bulbs
compared to even the best fluorescent tubes. Why
is this the case? Do they emit huge amounts of
UV? Are they reliable? Is one brand better than
another?

Self-ballasted MV lamps suffer from a 50% fail-
ure rate over the first six months and a 70% decay
rate in total UV-B emitted. The best-selling style
MV bulb, the 160 watt FLOOD, emits much less
UV-B than stated on their endorsements after
decay. These bulbs settle in at about 12 µW/cm2

at 12". Why, then, do we see such incredible results
with MV lamps when compared to fluorescents?
After all, two high-quality fluorescents will produce
25–30 µW/cm2 at 12". The answer is simple: MV

lamps emit less total UV-B than fluorescents — but
more “usable” UV. Studies have shown that MV
lamps produce the same percentages of D-UV (and
UV-A, which is another subject) as a percent of
total energy emitted as the sun.

Self-ballasted SPOT-style lamps produce much
higher UV readings than any other reptile bulb on
the market. They still are subject to the failure rates
of all self-ballasted MV lamps. These lamps settle
in at about 100–150 µW/cm2 (at 12"), but have
a much narrower disbursements of UV-B. These
are excellent rehabilitation bulbs for treating ani-
mals suffering from severe metabolic bone disease.
Regarding concerns that iguanas under this style
of lamp will need “goggles” to prevent blindness
from “excessive” UV exposure, remember the
exposure levels in their natural environment. We
have five iguanas that have spent two years exclu-
sively under mercury vapor SPOT lamps with no
negative impact on vision. However, the distance
from any MV lamp to the basking area must be
regulated in order to provide optimum tempera-
tures, regardless of any distances stated by the dis-
tributor!

84 IGUANA Volume 10, Number 3

ZooMed 5.0 fluorescent tubes are built to the most
exacting tolerances.

Erving, a rescued Green Iguana, made a remarkable
recovery following six months of therapy under UV lamps
(top). Note the severe spinal scoliosis (center) and evidence
of severe metabolic bone disease (bottom) when he was
initially rescued.



At this time I recommend only 100- and 160-
watt T-Rex UV Heat, 160-watt ZooMed
Powersun, and the Westron Lighting 60-watt in-
line ballast MV lamps. Others performed poorly in
my tests. A new 60-watt in-line ballasted MV lamp
from Westron Lighting produces excellent UV-B
after decay, and without the problem of failure (no
self-ballast to fail). Its primary drawback is limited
heat production, although this could be an asset
for keepers of smaller reptiles kept in glass habitats.
It also generates poor-quality visible light.
According to the manufacturer, these limitations
of the current bulb will not apply to models that
will soon be available.

So, what is best for iguanas? I recommend
30–50 µW/cm2 at 12" for 8–12 hours per day
(comparable to minimal natural exposures in the
wild). The best way to realize this number is to use
an ultraviolet radiometer (such as the Solar Meter
6.2 hand-held version; see references) to measure
the amount of UV-B available to your iguana. The
ZooMed 5.0 fluorescent lamps are at the top of
the ladder in terms of meeting these specific
requirements (i.e., the best “usable” UV-B). Use
at least two of these lamps to achieve the desired

exposure levels. The 100- or 160-watt T-Rex
FLOOD lamps or the 160 watt ZooMed FLOOD
lamp also may be used in conjunction with a
ZooMed 5.0 fluorescent tube. If you do not have
a meter, you should rely on the security provided
by the quality of the 5.0 bulbs. Alternately, T-Rex
SPOT bulbs (100- and 160-watt) will emit
50–150 µW/cm2 (at 12" after break-in) — as
long as they burn. No other supplemental UV-B
source is necessary with these lamps. Finally, the
Westron Lighting MV in-line ballast lamp can pro-
duce 30–50+ µW/cm2 after break-in. This lamp
will need additional heat and full-spectrum com-
plements, but should not fail as quickly or fre-
quently as other MV lamps.

References and Resources 
(listed by topic)

25 HDCC blood level study with radiograph (and much more
information on ultraviolet bulbs), http://www.myiguana.com

25 HDCC vitamin D Test, University of Michigan, Animal Health
Diagnostic Laboratory, PO Box 30076, Lansing, MI 48909
(517-353-0621)

Calcium physiology, Bogoslavsky, B. 2002. Calcium metabolism in
iguanas. Iguana Times (Journal of the International Iguana
Society) 9(1&2):32–34.

Electromagnetic spectrum,
http://csep10.phys.utk.edu/astr162/lect/light/spectrum.html

Photochemistry and biology, http://www.photobiology.com

Ultraviolet radiometers, www.solarmeter.com

UV-B and D3, http://home.att.net/~chameleons/zoomedUV-B.html

Vitamin D discussion, “The Merck Manual,” 17th ed., p. 35

85IGUANASeptember 2003

The Westron Lighting MV in-line ballast lamp requires
complementary heat and full-spectrum lighting but outlasts
other MV lighting.

ZooMed’s Powersun 
160-Watt lamp is another
lamp that performed well
when tested.

The T-Rex UV Heat 100- and
160-Watt lamps are among
those recommended.


