What is the germicidal UV wavelength commonly used for disinfection?

• A. 365 nm
• B. 254 nm
• C. 184 nm
• D. 400 nm

Answer: (B)

Germicidal UV disinfection relies on UV-C light because photons in this range are energetic enough to damage the genetic material of microorganisms. The most commonly used wavelength is about 254 nanometers, which falls in the UV-C band and aligns well with the strongest absorption of DNA and RNA. When DNA is exposed to this wavelength, thymine bases form dimers, disrupting replication and transcription and effectively inactivating bacteria, viruses, and protozoa. This is why standard low-pressure mercury lamps, which emit primarily at 254 nm, are widely used for disinfection in water, air, and surfaces.

Wavelengths in the UV-A range (around 365 nm) are much less effective for disinfection because they don’t cause as much direct damage to genetic material. The 184 nm vacuum UV can be used in some specialized processes and can generate ozone, but it’s not the general-purpose disinfection wavelength because it requires different materials and handling. 400 nm is visible light and does not produce the same germicidal effects.

So, 254 nanometers is the commonly used germicidal wavelength for disinfection due to its strong DNA/RNA absorption and practicality in standard UV disinfection systems.