Myth vs fact · Water
The claim: “A UV purifier is working as long as the lamp is glowing”
Does a glowing UV lamp mean your purifier is working?
The ruling
False, and it is the most under-recognised failure in Indian water treatment. The visible glow from a UV lamp comes from a different part of the spectrum than the germicidal UV-C output that does the disinfection. UV-C output falls to roughly 60–70% of new by around 9,000 operating hours — about a year of continuous running — while the lamp continues to light up normally. Almost no domestic unit measures actual UV output, so nothing anywhere in the system will tell you.
How this was checked
Settled by the underlying physics
Germicidal UV lamps emit principally at 254 nm, which is invisible. The visible light is incidental emission, and the two decay at different rates.
- 1Low-pressure mercury lamps used for water disinfection emit principally at 254 nm — well into the ultraviolet and entirely invisible to the eye.
- 2The blue-white glow visible through a viewing port is incidental visible-spectrum emission. It is not what does the disinfection, and its persistence tells you nothing about UV-C output.
- 3UV-C output declines through the lamp's life as the electrode and envelope degrade. Manufacturers typically rate lamps for around 9,000 hours to a stated end-of-life output, commonly 60 to 70% of initial.
- 4Disinfection is dose-dependent — intensity multiplied by exposure time. A 35% drop in intensity is a 35% drop in delivered dose at the same flow rate.
- 5A quartz sleeve accumulating hardness scale, iron deposit or biofilm attenuates transmission independently of lamp condition, and it is invisible from outside the unit.
- 6Turbidity shields organisms physically. BIS sets 1 NTU as the acceptable limit largely because disinfection efficacy falls sharply above it.
What is true in the claim
Every durable myth has a grain of truth, and pretending otherwise teaches nobody anything.
What it means for your purchase
- Change the UV lamp annually on continuous use, by date, regardless of whether it still lights.
- Insist that the quartz sleeve is cleaned at each service. It is a standard step and it is routinely skipped.
- If your water is cloudy, fix the sediment filtration before relying on the UV at all.
- If you are unsure whether the UV has been working, boil drinking water at a rolling boil for one minute until it is serviced.
What this page does not establish
- Lamp life figures vary by manufacturer and lamp type. The 9,000-hour figure is a common rating, not a universal one — check the specification for your unit.
- Some commercial and industrial UV systems include UV intensity sensors and do report output. This argument concerns domestic units, which almost never do.
Frequently asked questions
How often should I replace the UV lamp in my water purifier?
How can I tell if my UV purifier is actually disinfecting?
Sources
Every limit and standard quoted on this page comes from one of the documents below. Where a figure is not in one of these, we have not published it.
IS 10500:2012 Drinking Water — Specification (Second Revision)
Bureau of Indian Standards · published 2012-05-01 · checked 2026-08-01
Guidelines for Drinking-water Quality, 4th edition incorporating the 1st addendum
World Health Organization · published 2017-04-01 · checked 2026-08-01
These pages check claims against published standards, the underlying physics, or our own testing where it exists — and each page says which. We do not emit ClaimReview structured data, because that markup is reserved for accredited fact-checking organisations and we are a commercial publisher. Last reviewed 2026-08-08.