It is inevitable given the fast pace and, in some instances, hype of the news media that some will be curious about the risks of contracting COVID-19 (the official name of the “disease” caused by the virus) from the spread of the SARS-CoV-2 coronavirus (the official name of the viral pathogen) in a swimming pool environment. This is a complicated subject and this post does not serve as a substitute for appropriate medical advice that one would get from their family physician. As well, it is always best to check in with the latest updates, FAQs, and recommendations from the US Centers for Disease Control (CDC) here –
Whenever we talk about pools, disease transmission and disinfection, we must always try to maintain a clear understanding of the environment that is being considered and know that no two pools or swimming environments are the same. In general terms, a single-family residential pool that is properly maintained and uses one of the three EPA-recognized pool sanitation methods (chlorine, bromine, or biguanide) is a very low risk environment for disease transmission. If no one but immediate family members use the pool, then you are not generally exposed to any new illnesses that are not already part of your normal family environment. A public pool or large pool party, on the other hand, definitely increases your exposure risk to new pathogens in ways that are very similar to how community transmission of diseases occurs – you are surrounded by a large cohort of people that you are not intimately familiar with nor do you know their health status. Therefore, in public environments, you are at a much higher risk of contracting a disease-causing pathogen simply from a higher level of exposure. Limiting your family’s exposure to public venues is certainly one way to lower your overall risk of contracting an illness.
Just as no two swimming pools are created equal, no two pathogens are the same either. We will limit the discussion to disease-causing pathogen, bacteria and viri (“viri” is the plural form of “virus”) because algae, while a nuisance to pools, is rarely a disease-causing organism (some forms of algae can produce exotoxins that will make humans and animals sick, but that is a different subject matter). Even within the broad categories of “bacteria” and “virus”, there is a lot of variation to the effectiveness of disinfectants and, in the case of spore-forming bacteria, there can be nearly total resistance to standard disinfecting agents. For the sake of brevity, the following will focus simply on what one can or should expect chlorine-based disinfectants to do with respect to the SARS-CoV-2 coronavirus.
Disinfection for Swimming Pools
There is some good information on disinfection as it relates to coronavirus (see this web link - https://www.uptodate.com/contents/coronaviruses#H13). The disinfection times and log reduction of coronavirus is applied to hard surfaces that one might find in a hospital setting, so you can’t draw a clear distinction straight to a pool. As it turns out, the coronavirus can be quite viable outside of the human body and resistant to disinfection, but it does require that the virus be in lower temperature and higher humidity environment. Once desiccated (no water surrounding it), it loses its virulence. What this means for pools is not so clear but it’s not unreasonable to assume that pool water does not make a good environment for this virus to be in. Pool water with adequate levels of chlorine and exposure to UV light from the sun is going to have a very high redox potential which will help to neutralize pathogens quickly. There is far more risk of contracting the coronavirus through direct person-to-person transmission (sneezing or spitting water in people’s faces) or from sharing hard surfaces that may have been in contact with an infected individual.
When epidemiologists look at pathogens and disinfectants, the standard concept used is in determining effectiveness is the decimal reduction time – in other words, the contact time of the disinfection process needed in order to reduce the colony size by a factor of 10 (or a 90% reduction). This is also referred to as the “D-value” of a particular disinfection process whether that be thermal, high-pressure, chemical, or radiological in nature. When chemical disinfection is employed, there is an additional variable to consider – the concentration of the chemical disinfectant in solution (C). It is standard practice for epidemiologists to report their findings as some specific reduction in pathogen level (for example, 3-log reduction which is 99.9%) for a given a CT factor (concentration x contact time, typically reported as mg/L•min). So, for example, one might compare the effectiveness of chlorine versus chloramine in killing Escherichia coli by looking at the time it takes for a 2-log (99%) reduction in the number of colonize on a growth plate when the concentration of each sanitizer is varied over a specific range.
Measuring the effects of a disinfectant on bacteria is fairly easy since bacteria are self-contained organisms. They have within them all the cellular machinery needed to replicate and grow. One can simply take a culture of bacteria, spread it onto a laboratory growth plate (often an Agar gel plate) and then expose that plate to different chemical agents. Bacterial will often form colonies that are easily counted under a microscope and then the effects of a disinfectant can be readily evaluated.
Viri, on the other hand, are fairly difficult to measure for several reasons. First, an individual virus is not so easily characterized as a “living” organism. In their simplest forms, a virus is nothing more than a protein shell with DNA, or RNA, on the inside and some surface proteins (often called spike proteins) on the outside to help the virus connect to and infiltrate host cells. A virus is tiny as well, most often too small to see with a standard optical microscope. On its own, the virus can do nothing; it requires a host cell in order to replicate and that occurs when the virus has entered the body and can then latch onto cells (usually somewhere in the respiratory track as the most common route of infection). So, there is no simple way to “plate” a virus, watch it grow, and count its numbers. Each virus is different and may require specialized growth medium with live cells in it to allow the virus to replicate or one must use an animal model, like a rodent, to receive the infection. In these ways, there is no simple or easy way to measure how many viri are created and often one has to look at virulence, the ability of a virus to cause a known infection with a specific set of symptoms or measurable biochemical markers (antibodies). Typically, one exposes a sample of the virus to a disinfectant and then looks to see if that exposed viral load can now cause an infection. If there are measurable biochemical markers, like antibodies, then one can determine how virulent the degraded pathogen is. If there’s no easy way of testing for antibodies, then often one has to simply rely on the infection causing symptoms in an animal subject and then determine at what level of exposure to disinfectant did the virus stop causing sickness. These studies can be very complicated and require significant time and effort to do. Therefore, data on actual viral inactivity is often hard to come by except in the most extreme cases.
This link from the World Health Organization is a good primer on how inactivation processes work and are measured - https://www.who.int/water_sanitation_health/water-quality/guidelines/en/watreatpath3.pdf?ua=1
Suggestion for your pool
Given all of the available data out there the most reasonable course of action is to simply keep your pool clean and sanitary by following proper FC/CYA ratio guidelines and, as is always the case, limited large-scale interactions with persons outside your immediate family. If you know someone is sick, no matter the illness, they really should not be swimming in anyone’s pool. If you suspect that a person has been in your pool and that they may be a carrier of the illness, prudence would suggest performing a SLAM of the pool for a 24 to 48 hour time period to allow the higher levels of chlorine the time needed to inactivate pathogens.
Some users may wish to stock up on Liquid Chlorine fearing a rush from this outbreak. This idea is completely understandable, but users should be mindful that bleach does breakdown over time. Users who do stock-up should store their chlorine in a cool dark area to help reduce the speed the chlorine's strength is reduced.
And, lastly, we are headed into summer and the weather is going to get a lot warmer. Assuming this virus follows the patters of other common viruses, the coronavirus, like so many others, may not survive well outside the human body and once surface temperatures exceed 80F, their virulence diminishes rapidly. This is why colds and flu’s are so prevalent in the winter months – the temperatures tend to favor transmission of the disease through surface contact as well as person-to-person transmission.
Now go out and enjoy your pool and, if you’re still anxious, pick up your favorite adult beverage and enjoy a cool sip by the pool….