It's a common belief that bright, sunny days lead to more chlorine loss in swimming pools than cloudy, rainy days. But is this really the case? We set out to investigate this theory. This article explains what UV does to chlorine, and the impact of clouds and cyanuric acid on chlorine photolysis.
Covered in this article:
- Ultraviolet (UV) light and wavelengths
- Effect of cloud cover on solar UV radiation
- Cyanuric acid vs. UV light
- Water temperature does not impact UV degradation of chlorine
- UV as a secondary disinfectant
- Conclusion
Ultraviolet light and wavelengths
Ultraviolet (UV) light is a type of electromagnetic radiation that is categorized into three main types based on wavelength: UV-A, UV-B, and UV-C.
UV-A rays have the longest wavelength and are the least harmful to the skin. They comprise most of the UV radiation that reaches the Earth's surface and can penetrate deep into the skin, causing premature aging.
UV-B rays have a shorter wavelength and are more damaging to the skin. They are the primary cause of sunburns and are also linked to the development of skin cancer over time.
UV-C rays have the shortest wavelength and are the most harmful to living organisms. Fortunately, the Earth's atmosphere blocks most UV-C radiation from reaching the surface.
Free chlorine (particularly OCl-) is destroyed by UV-B rays with a wavelength of around 295nm.1,2,11 One study examined the impact of UV-C light at 254 nm on free chlorine, and found that it destroys some chlorine too...but not much.3
This is significant because 254 nm is the wavelength of UV light produced by low-pressure UV disinfection systems used in drinking water and swimming pools. In our experience with hundreds of swimming pools that use low-pressure UV systems, our customers have not told us they see a noticeable reduction in free chlorine. Medium-pressure systems, however, do reduce chlorine more noticeably.
In water, UV light can have both positive and negative effects. On one hand, it can be used as a water disinfection method, deactivating microorganisms and preventing the spread of waterborne diseases.4 On the other hand, UV radiation can also destroy chlorine in a process called photolysis.2,3,5
Effect of cloud cover on solar UV radiation
Clouds are made of condensed moisture that can refract light and disperse it. This refraction can reduce the intensity of the UV rays hitting our skin and the pool's surface. But while clouds provide some level of protection against UV rays, they do not block them completely.
Contrary to popular belief, chlorine can still be lost to sunlight on cloudy days. The UV radiation that destroys chlorine can penetrate clouds and bad weather. If you've ever been sunburned on a cloudy day, you may already understand this.6
The amount of UV light that penetrates clouds depends on various factors, such as their thickness and type. Thicker clouds can block more UV radiation, while thinner clouds may allow more UV radiation through.7
Cyanuric Acid vs. UV light
We have many other articles about CYA in our blog and help center, so we will only do a quick summary here.
Cyanuric Acid (CYA) is used in swimming pools to bind with chlorine to protect it from sunlight degradation. Chlorine bound to CYA is called stabilized chlorine, or more technically, chlorinated isocyanurates.
Without CYA in the water, chlorine degrades quickly. Unprotected chlorine's half-life is only about 20-45 minutes, meaning about 75% of chlorine will be gone in just two hours.5,8,15 Good luck holding a free chlorine residual for an entire day in an outdoor pool.
Chloroisocyanurates are substantially more resilient against UV degradation than HOCl or OCl-. Without CYA, sunlight destroys chlorine in the following reaction:
2HOCl + UV🌤️ → 2HCl + O2
2 Hypochlorous acid + UV light → 2 Hydrochloric acid + Oxygen
This process also lowers the pH, because chlorine converts into hydrochloric acid. This is why hypochlorite chlorines (like liquid chlorine and cal hypo) only temporarily raise pH in water.
Source: Robert W. Lowry 9
Water temperature does not impact UV degradation of chlorine
Pool professionals and owners correctly recognize that hotter days make it more difficult to hold chlorine. Unfortunately, this leads to many believing that more chlorine is being lost to sunlight degradation on hotter days. But that's not what's really going on.
Coincidence is not the same as causality. There's a difference between using chlorine and losing chlorine.
Warmer temperatures mean the water is using chlorine faster; it's performing faster, there are more contaminants to oxidize and kill, and more people are likely using the pool when it's warmer.
Losing chlorine refers to sunlight degradation (photolysis). When CYA is in the water, its ability to protect chlorine from sunlight has virtually nothing to do with water or air temperature. Temperature's role is negligible in photolysis.10 Learn more here.
UV as a secondary disinfectant
UV light has been proven to be highly effective in breaking down chloramines and destroying germs and viruses.11,12 The optimal disinfection wavelengths are shown in the chart above. The sun throws out electromagnetic radiation across the light spectrum, but the question is whether or not those rays are intense enough to disinfect and destroy chloramines.
The answer seems to be no.13 The study cited earlier (3) introduces two concepts called "quantum yield" and "fluence".
"Fluence refers to the total radiant energy passing through a small transparent imaginary spherical target containing the point of interest, divided by the cross-sectional area of the target." 13
While sunlight rapidly decomposes non-stabilized chlorine, combined chlorine (i.e. waterborne monochloramines and dichloramines) is much more stable. You may have noticed this if your pool has had combined chlorine before. Chloramines stay in the water much longer than chlorine, even in the presence of sunlight.
If sunlight alone could effectively disinfect microbes, life on Earth would cease to exist. Our Atmosphere gives our planet the unique ability to withstand the intensity of the sun's rays. Concentrated UV light, however, has more fluence (concentrated power), allowing it to disinfect pathogens in water and destroy monochloramines.
Conclusion
The sun emits light across most of the electromagnetic spectrum. Wavelengths shorter than visible light are called Ultraviolet (UV), and those rays can penetrate clouds. UV rays can also rapidly decompose non-stabilized free chlorine in swimming pools.
While clouds deflect and diffuse UV light, they do not block it completely. UV rays still get through clouds, and can still destroy chlorine in pools, even if it's raining. Just like humans can be sunburned on a cloudy day.
The main takeaway here is that minimal cyanuric acid in the pool is very beneficial. Just don't use too much of it so you can avoid the problems associated with overstabilization.
1 Several credible sources differ on the exact wavelength for chlorine destruction. If you explore these sources, they refer to chlorine destruction in terms like "HOCl photodecay"5 or "photochemical reactions that dissociate free chlorine to form hydrochloric acid."6 Some sources say it's between 180-200 nm, but most indicate between 245-365nm for destruction of HOCl. This is why low-pressure UV systems do not reduce chlorine levels, whereas medium-pressure UV systems can. In our graphic, we mark the destruction of HOCl at 295 nm, based on the EPA.10
2 Yin, R., Ling, L., Shang, C. (2018). Wavelength-dependent chlorine photolysis and subsequent radical production using UV-LEDs as light sources. Water Research. Vol. 142 (pp. 452-458).
3 Feng, Y., Smith, D., Bolton, J. (2007). Photolysis of Aqueous Free Chlorine Species (HOCl and OCl-) with 254 nm Ultraviolet Light. Journal of Environmental Engineering and Science. 6(3): 277-284.
4 McClean, Jon. (2021). Using UV for Dechlorination. Wastewater Digest.
5 Nowell, Lisa N., Hoigné, Jürg. (1992). Photolysis of aqueous chlorine at sunlight and ultraviolet wavelengths––I. Degradation rates. Water Research. Vol. 26 (5), pp. 5993-598.
6 Venosa, Ali. (2022). 5 Sneaky ways you're being exposed to the sun's UV rays. Sun & Skin News.
7 Schoonmaker, David. (2006). Sunshine on a Cloudy Day. American Scientist. Vol. 94 (3), pg. 217.
8 Lowry, Robert W. (2016). IPSSA Basic Training Manual (2016 Revised Edition), pg. 108.
9 Lowry, Robert W. (2018). Pool Chemistry for Residential Pools. Pool Chemistry Training Institute (PCTI). Ch. 5, pg. 46.
10 Furatian, L., Mohseni, M. (2018). Temperature dependence of 185 nm photochemical water treatment - The photolysis of water. Journal of Photochemistry and Photobiology A: Chemistry. Vol. 356, pp. 364-369.
11 US Environmental Protection Agency (1996). Ultraviolet Light Disinfection Technology in Drinking Water Application: An Overview. NEPIS.EPA.GOV.
12 Cimetiere, N., De Laat, J. (2013). Effects of UV-dechloramination of swimming pool water on the formation of disinfection by-products: a lab-scale study. Microchemical Journal. Vol. 112 (pp. 34-41).
13 Örmeci, Banu & Ducoste, Joel & Linden, Karl. (2005). UV disinfection of chlorinated water: Impact on chlorine concentration and UV dose delivery. Journal of Water Supply: Research and Technology - AQUA. 54. pp. 189-199.
14 Lam, H. (2002). Solar-terrestrial Magnetic Activity and Space Environment. COSPAR Colloquia Series.
15 The rate of chlorine decomposition depends on many factors, including water depth. the deeper the pool, the slower the decomposition process of the chlorine in the deep end. This is due to light refraction by the water. Because of this variable alone (though there are others), it's impossible to know an exact half-life of HOCl in sunlight.