UVB LIGHT AND VITAMIN D3
Ultraviolet light is electromagnetic radiation that exists in three main bands: UVA, UVB, and UVC. Each band is characterized by a specific wavelength. UVA has the longest wavelength at 320‐400nm while UVB has a wavelength of 290‐320nm. Both UVA and UVB have low tissue penetration with most light being absorbed at the epidermis. UVA has a wavelength of less than 290nm, the deepest penetration, and is the most harmful. These wavelengths place ultraviolet somewhere between visible light (369‐769nm) and X‐rays (0.01‐10nm). The ozone layer is responsible for filtering the majority of UV radiation hence protecting living organisms from its harmful exposure. UVC is considered germicidal and can have significant harmful effects to living animals while both UVA and UVB have therapeutic and physiologic applications. Still, as we know from human medicine, too much sun/ultraviolet exposure has a slew of negative implications ranging from sunburn to premature aging, and cancer.
UVB light is essential for the synthesis of vitamin D3. Although commonly referred to as a vitamin, D3 is actually a hormone. UVB light from the sun stimulates conversion of 7‐dehydrocholesterol to cholecalciferol (D3) in the skin. Cholecalciferol is then transported to the liver via the blood stream and hydroxylated into 25‐hydroxycholecalciferol (calcidiol), which is then transported to the kidneys for hydroxylation into 1,25‐dihydroxycholecalciferol(calcitriol), the active form of D3. The 25‐hydroxycholecalciferol in the liver is the storage form of D3 and is a natural source of D3 for animals fed whole prey. Plants are a source of ergocalciferol or ergosterol (D2), which is not readily bioavailable to most reptiles and is therefore not an appropriate source of D3 supplementation. Calcidiol, an animal source of D3, is more bioavailable for reptiles.
There are two main sources of UVB, natural sunlight and UVB bulbs. Exposure to natural sunlight is the most efficient way for reptiles to produce adequate levels of D3. However, this is often not possible year round, outside the tropics and some subtropical regions, necessitating the use of artificial UVB light sources.
CHOOSING UVB LIGHTING
There has been a long‐standing debate about the effectiveness and even potential harmful effects of artificial UVB lights. Like the application of any treatment or therapeutic, the use of UVB lights must be in accordance with certain parameters for it to be effective and safe. Nowadays there are high‐quality bulbs in the market that provide adequate UVB while also being safe when used appropriately. UVB bulbs can be further divided into linear fluorescent, compact fluorescents, and mercury vapor. In the future, commercial LED UVB lights may also become available. Fluorescent UVB bulbs provide light in the required wavelength for D3 synthesis but no heat. Mercury vapor bulbs provide both heat and UVB.
There are six main aspects to consider when selecting a UVB bulb:
1 Size/type of the enclosure.
2 Type of material between the bulb and the reptile.
3 Type of bulb.
4 Effective distance of the bulb.
5 Bulb longevity.
6 Species of reptile.
Another consideration is the type of fixture used for the bulb. There are a number of reptile‐specific fixtures that offer the advantage of having reflective materials to increase light reflection and decrease scattered light emission (radiation). Some also have a ceramic base and are more tolerant of higher temperatures produced by mercury vapor bulbs. Reptile‐specific light fixtures or those that reflect light away from the bulb and towards the animal are preferred over plastic fixtures or those without reflective properties.
T8 Linear Fluorescent Tubes
T8 bulbs (25‐mm or 1‐inch diameter) are available in different lengths and fit into standard aquarium or reptile hoods. Some manufacturers further classify their bulbs by a number system ranging from 2.0 to 10.0. This number represents the percentage of light that is emitted in the UVB spectrum. A 5.0 has 5% of the emitted light in the UVB spectrum; a 10.0 has 10% UVB emission, and so on. Arcadia® has simplified the process by simply stating the percentage of UVB emission with their bulbs ranging from 6% to 14%.
Being a longer bulb also means there is more scattered radiation so the UVB exposure is not as focused. One recommendation is that the longest bulb possible be used to cover a wide area of the enclosure. An alternative is to use a shorter bulb in combination with a heat lamp while leaving the rest of the cage shaded. These bulbs emit UVB light at an average effective distance of 12–20 inches, depending on the brand and percentage of emitted UVB. It is generally recommended that animals should not be abe to get any closer than within 3– 4 inches of bulb’s surface. Bulbs need to be replaced every 6–12 months according to manufacturer’s recommendations.
T5 High Output Linear Fluorescents
A high‐output light is a T5 bulb (16‐mm or 0.6‐inch diameter) that requires a hood with a T5 fixture. T5 bulbs are more efficient and provide a higher amount of UVB than T8 or compact fluorescent bulbs. In theory a T5 10.0 bulb produces a higher amount of UVB than a T8 10.0 or compact fluorescent. It is worth mentioning that Zoo Med Laboratories Inc. produces a T8 10.0 bulb labeled high output but their T5 10.0 bulb is actually a much stronger bulb and is considered as a true high output for our purposes. These bulbs emit UVB light at an effective distance of 12–36 inches, depending on the brand and percent of emitted UVB. While it is generally recommended that animals should not have the ability to get any closer than within 3–4 inches of bulb’s surface, this distance may actually need to be longer for high output bulbs. They must be replaced every 9–12 months according to manufacturer’s recommendations.
Compact Fluorescent
Compact fluorescent bulbs can be fitted into any incandescent fixture that would fit a household bulb but a reflective or reptile‐specific fixture is preferred. The light is focused over a smaller area so there is less scattered radiation. For this reason, it is possible that a 10.0 compact fluorescent may actually emit a higher UVB output than a 10.0 linear fluorescent over the same area. It is usually recommended to place these bulbs next to the heat lamp so the reptile gets UVB exposure while basking. All other properties are similar to the linear fluorescent bulbs although some may think of these as being less effective and being “weaker” in the UVB spectrum.
Mercury Vapor Bulbs
Mercury vapor bulbs have been successfully used by breeders and zoological institutions for decades but have just recently made it into the mainstream exotic pet market. Their external appearance is that of a floodlight and they are commonly found as a self‐ballasted variety in the pet trade. Self‐ballasted bulbs can be fitted into an incandescent fixture that would fit a household bulb but a reflective or reptile‐specific fixture is preferred. A fixture with a ceramic base is also recommended because of the high amount of heat the bulbs produce. The light is focused over a shorter area so there is less scattered radiation. Their UVB output is based on the wattage of the bulb and the fixture should accommodate the appropriate wattage. Their effective distance is 12–36 inches. There are also some high‐output bulbs with an effective distance of up to 8 feet. A major advantage is the production of heat and a stronger UVB output compared with all other bulb types. In the hands of inexperienced individuals, however, these bulbs could be detrimental to reptiles.