Residual Biocidal Activity – is the confusion due to QACs?
The use of disinfectants based on quaternary ammonium compounds (QACs) has played, and will continue to play, a massive role in measures designed to minimise the spread of SARS-CoV-2, the Coronavirus responsible for the human disease COVID 19.
But what is a QAC chemical?
QACs belong to a group of surfactants in which substantivity – the ability to adhere to surfaces - is a typical feature, and has been exploited to great effect, and commercial gain, for at least 50 years in the form of fabric conditioners.
The biocidal properties of QACs (i.e. the ability to destroy bacteria and viruses) were established in the 1930s and have also been successfully exploited to the present day. So, it comes as no surprise that residual biocidal activity attributable to the substantivity of QACs, has long been reported.
It is, therefore, not unreasonable to anticipate that some prolonged benefit might arise from the use of such products in hygiene measures adopted to combat the spread of Coronavirus. A large number of claims have been made to that effect, by companies wishing to promote their products in a wide range of environments.
Editorial update: 25 October 2026. This article was originally published during the COVID-19 pandemic. Residual-protection and fogging guidance has been corrected below; follow current product instructions and the relevant site risk assessment.
A disinfectant's residual efficacy and the duration of any protection are product- and use-specific claims. An ingredient's ability to adhere to a surface does not, by itself, demonstrate a stated period of protection in everyday use. Ask the supplier for evidence supporting the precise claim, including the organisms, surfaces and test conditions covered.
For a claim such as protection for 30 days, check the product label and supporting efficacy information rather than assuming that all QAC formulations behave alike. Consider how cleaning, abrasion and recontamination affect the conditions in which the product will be used. A general disinfectant test result alone should not be read as a guarantee of prolonged protection.
Fogging needs a separate safety assessment. HSE's guidance on disinfectants in the food and drink industry identifies particular respiratory risks from fogging and calls for a COSHH assessment and controls to prevent premature re-entry. Read the product label, safety data sheet and application instructions before choosing a method.
Hypochlorous Acid should not be described as unconditionally safe to fog. The active ingredient alone does not establish that a particular formulation, concentration or application is safe or effective. Use only an application supported by the product's instructions and the site risk assessment, with the required exposure and re-entry controls.
Do not use an unsupported residual-protection claim to justify reducing routine cleaning. Set a documented cleaning and disinfection programme appropriate to the setting, and review it against product instructions and the relevant infection-prevention requirements.
The most consistent piece of advice that has been offered by government and scientific advisers through the course of the pandemic has been to wash your hands frequently; ahead, even, of advice to apply hand sanitiser, placing the emphasis on removal of infective material, instead of deactivation.
Continue removing dirt and contamination through an appropriate cleaning programme. Where disinfection is needed, follow the specified dilution, contact time and application method. Treat a residual-efficacy claim as something to substantiate for the intended use, rather than a substitute for routine hygiene.
