By Doug Collins, Assistant Professor of Chemistry, Bucknell University (@EarthMechanic)

Mosquito fogging
Mosquito fogging – CC Wikimedia Commons

We have known for nearly a century that ‘germs’ (viruses, bacteria, and other microbes) are the causes of infectious disease. Most of the diseases that we worry about on a regular basis can be slowed or stopped by washing our hands, cleaning surfaces, and staying away from sick people until they fight off the illness and feel better again. Apart from staying isolated, the other two ways we keep ourselves safe involve disinfecting our hands or surfaces around us. Many people have taken this knowledge full-force into the COVID-19 pandemic, making surface cleaning supplies and hand soap very hard to find! What we’ve been learning, however, is that SARS-CoV-2 (the virus that causes the disease called COVID-19) can be spread through the air and we can be infected by inhaling airborne particles that contain the virus. So how do we disinfect the air? What if we took our surface cleaning chemicals and sprayed them into the air — that should work, right? Well. Maybe. But maybe there are reasons not to spray chemicals everywhere or maybe there are things we should know before doing so.

First, spraying disinfectant outdoors for the purpose of disinfecting air for SARS-CoV-2 is really not worthwhile. The outdoor air will spread out any airborne virus particles faster than spraying could do the job and sunlight is likely a good killer of the virus — so we will focus on air indoors. There are a few kinds of chemicals that you might find being sprayed indoors. Chemicals often called “quats” (or quaternary ammonium compounds) are potent disinfectants. These compounds are very heavy (for molecules) and they are ions, so they do not tend to evaporate much — they’ll stay dissolved in liquids, liquid fog droplets, or coated on a surface. There are some concerns about side effects of using quats, but there needs to be more thorough study of these products to be sure. The other chemical you might find in a sprayer or fogger can be broadly called “active chlorine”. This term represents a group of compounds, mostly including molecular chlorine (Cl2), sodium hypochlorite (NaOCl), and hypochlorous acid (HOCl), although chlorine dioxide (ClO2) is also used on occasion. The sprayer might say that it includes one of these or all of them — but unless great care is taken with handling active chlorine solutions, a mixture of HOCl, NaOCl and Cl2 are likely to all be present to some degree in water. A solution of sodium hypochlorite (NaOCl) in water is better known as ‘chlorine bleach.’ When the pH of a chlorine bleach solution, which normally has pH of about 11, is decreased to become more acidic to a pH between 4 – 7, the hypochlorite will change over to hypochlorous acid (HOCl).

Hypochlorous acid causes us to stop and think for two reasons. First, HOCl is an extremely potent disinfectant. Many researchers think HOCl is actually the compound that does the ‘dirty work’ of killing microbes when we use chlorine bleach. Some companies have found ways to make HOCl directly (without the sodium hypochlorite, so the solution is not “bleach” exactly) and are marketing it as a different and totally safe cleaning solution. To be clear, the US FDA and US EPA have approved HOCl solutions for cleaning surfaces, food, and even skin, wounds, and other body parts during medical procedures. For surface disinfection to work, the solution normally needs to be in contact with the surface for 1 minute or more. There are more than 10 HOCl solutions on EPA’s List N, which tabulates the products approved for use against the SARS-CoV-2 virus on surfaces.

That brings us to the second reason that HOCl should make us stop and think: unlike sodium hypochlorite, HOCl can evaporate out of a solution and exist as a gas. When we mop a floor with bleach (as done in the HomeCHEM study) a large quantity of HOCl is released into the air — but actually, that’s not all. HOCl is a potent disinfectant because it is highly reactive – it is a strong oxidizer. Many gaseous HOCl molecules indoors will find gaseous ammonia (NH3) molecules that to react with to form compounds called chloramines, which are actually the chemicals that “smell like bleach”. In reality, chloramines are quite bad for our health. Prolonged and repeated exposure to bleach fumes (studied as a mixture of all gases that bleach creates) has been associated with negative effects on the respiratory systems of cleaning professionals (see this review of health research on spray disinfectants). Truth be told, we need to do a lot more research on this topic, because not enough is known about these common household chemicals. What we do know gives us cause for concern and cause to limit our exposure by avoiding the products, use them when unprotected people are present, and/or aggressively bring fresh air into the spaces where they are used before people come back.

To be clear, any time HOCl is found in water, some of the HOCl molecules that are present will evaporate and then will be found as a gas until they find some other chemical to react with or until they are removed by ventilating the space with fresh air.  In many cases, products marketed as HOCl say that they “are not bleach.” That’s true in some ways, but HOCl is found in bleach, is found in the air just above bleach solutions, and is the most potent compound in the solution! At the same time, to my knowledge, the effects of inhaling HOCl have not been thoroughly studied. Claims of safety for inhalation are often based on indirect evidence or safety studies done for skin contact or ingestion — but not direct inhalation studies. Be cautious and skeptical of safety claims, especially when the producer does not provide evidence to support them. The EPA has not yet approved (as of December 15, 2020) all or most solutions for fogging and air disinfection. Never use a product for fogging or spraying in any application unless the product’s label has clear instructions on how to do so.

If you feel that you must use HOCl to clean an indoor space, please consider the following:

  • Do not spray disinfectant solutions into the air while people are inside the space. Do not re-enter the space for at least 20-30 minutes after spraying.
  • Most disinfectant solutions are only proven to be effective, and are only approved, for cleaning surfaces unless specifically labeled otherwise.
  • Instead of spraying disinfectants to clean the air, consider installing an air cleaner that uses HEPA filters or a fan-filter box and use them continuously.
  • Cleaning surfaces is important, but it’s not the end-all, be-all for reducing risk of spreading SARS-CoV-2. It is just one of the Swiss cheese slices in a stack.