Different approaches to large-scale disinfection have been used in response to COVID-19 but chlorine spraying, which is often implemented in response to cholera outbreaks, appears to be the most common approach.
What is chlorine spraying?
Chlorine spraying is usually done by professionals in patients’ houses or public spaces and involves applying a chlorine solution onto target surfaces without touching them. A wide range of equipment may be used, from traditional spraying backpacks, which tend to generate relatively large droplets of disinfectant, to high-pressure fogging machines, which typically produce finer droplets (fog or mist), as illustrated in this video.
What are the recommendations?
Chlorine spraying for disinfection as a COVID-19 response activity in households or public spaces is currently not recommended by UNICEF and WHO. Spraying of individuals is also not recommended in any context as it would not reduce an infected individual’s ability to transmit COVID-19 and chlorine inhalation or skin exposure to the spraying solution could be harmful.
Chlorine at a concentration of 0.1% can inactivate coronaviruses on surfaces within 1 minute (see “How resistant is SARS-CoV-2 to disinfection?”). However, application of chlorine by spraying/fogging may not be effective due to:
Chlorine losses through volatilization during spraying (in the air, before the generated mist or fog reaches the target surface);
Presence of dust or soil on surfaces in absence of cleaning, which can use up chlorine applied for disinfection and/or physically protect microorganisms such as viruses;
Incomplete surface coverage by spraying and thus incomplete disinfection - for example, surfaces that are shielded by objects or folded fabrics are unlikely to be properly disinfected;
Varying spray characteristics, as different equipment will generate sprays with different properties which may be more or less effective. This laboratory study suggests that spray velocity may affect disinfection effectiveness.
Additionally, potential drawbacks of large-scale chlorine spraying include:
Exposure to chlorine during spraying may cause adverse health effects, including eye, skin and respiratory irritation;
Large-scale disinfection programmes are resource-intensive and can be logistically challenging to implement;
Household disinfection should be carried out as soon as possible after the contamination of surfaces. Yet mobile disinfection teams may not be able to identify households with COVID-19 patients in a timely manner, due to the long (up to 14 days) pre-symptomatic phase of the infection, and high number of asymptomatic individuals. This reduces the effectiveness of home disinfection;
Household spraying during cholera outbreaks can cause stigmatisation towards case households and damage household possessions. These considerations may be relevant to the COVID-19 pandemic as well;
Spraying could create a false sense of security, as people might reduce precautionary practices in areas sprayed with chlorine, even though the spraying might not have effectively decontaminated the area.