Practice question

Ultraviolet germicidal irradiation emitting at 254 nm wavelength is installed in many biosafety cabinets and transfer hoods as adjunct surface decontamination method. Photons absorbed by nucleic acids induce formation of cyclobutane pyrimidine dimers and 6-4 photoproducts between adjacent thymine or cytosine residues, blocking DNA polymerase progression and transcription, thereby inactivating bacteria, mycoplasma, fungal spores and viruses deposited on stainless steel or plastic work surfaces. Decontamination occurs only on directly illuminated surfaces with sufficient intensity and exposure time, typically 30 minutes when cabinet empty, blower off, and no personnel present because UV causes skin erythema, keratitis, and accelerates plastic degradation. UV does not sterilize liquids, penetrates poorly through organic debris, does not provide white light for microscopy, does not heat samples, and cannot replace HEPA airflow. Modern BSL-2 guidelines emphasize chemical disinfection with 70% ethanol or quaternary ammonium compounds as primary method, recommending UV only as secondary measure and advising against reliance for sterility assurance due to shadowing effects.