When to Replace UVB Bulbs: The Schedule That Prevents MBD
UVB bulbs keep producing visible light long after their UVB output has faded, and a spent bulb quietly leads to metabolic bone disease. This checklist explains when to replace UVB bulbs, how to track their age, and how to confirm output with a meter.
Quick answer
Replace most UVB bulbs on a schedule, not when they stop glowing. As a general rule, replace T5 tubes about every 12 months and T8 tubes and compact UVB lamps about every 6 to 12 months, following the manufacturer's stated lifespan. The light stays on long after useful UVB has faded, so track the install date and, ideally, verify output with a UV meter.
UVB bulbs keep producing visible light long after their UVB output has faded, and a spent bulb quietly leads to metabolic bone disease.
Why a spent UVB bulb is dangerous
Most diurnal reptiles need UVB to make vitamin D3 and absorb calcium. Without it, they slide into metabolic bone disease (MBD): soft bones, deformities, tremors, weakness and, untreated, death. The cruel part is that a UVB bulb keeps emitting normal visible light long after its UVB output has dropped too low to keep an animal healthy, so the enclosure looks fine while the reptile slowly declines.
Label each UVB bulb with its install date so replacement is never left to memory.
A simple replacement schedule
Follow the lifespan printed on the packaging, and when in doubt replace sooner rather than later.
These are general figures. Always defer to the specific manufacturer's guidance for your exact model, as quality bulbs from reputable brands often last longer than cheap ones.
Track the age of every bulb
The most common failure is simply forgetting when a bulb went in. Build a habit so it never depends on memory.
Verify output with a meter
A schedule is a safety net, but a UV index meter (such as a Solarmeter) tells you what the bulb is actually producing at basking height. This is the gold standard, especially for keepers of species with high UVB needs.
A UV index meter is the only way to know a bulb's real output, since UVB fades invisibly.
Quick FAQs
My UVB bulb still lights up. Do I really need to replace it? Yes. Visible light is not UVB. Output fades invisibly, so a bright-looking bulb past its lifespan may give almost no useful UVB.
How can I tell if UVB has faded without a meter? You largely cannot by eye, which is why the install-date schedule exists. A UV meter is the only reliable way to measure output directly.
Does a UVB bulb behind glass or mesh work? Glass and most plastics block UVB, so mount the bulb over an open mesh section. Fine mesh still reduces UVB, so account for it when setting height.
Is more UVB always better? No. Both too little and too much UVB cause problems. Aim for the target UV index range for your species at the basking spot, not the maximum possible.