Toxicology and swimming pool management are both applied forms of chemistry. My six summers as a lifeguard and pool operator prepared me to be a doctor as well as any class in medical school (I’m looking at you, histology.) I now work in an emergency department and specialize in toxicology. And, while I long ago traded my whistle for a stethoscope, the jobs are more similar than they would appear on the surface.
I was a rising high school senior in 1993 when I started my first season as a lifeguard. I had completed the requisite American Red Cross course and a job interview, which mainly consisted of 500 yards of freestyle. Walking onto the concrete pool deck on my first day, I was like a shiny medical intern; I had the required training but very little real-world experience. Lifeguard training doesn’t tell you how to spot the one drowning kid in a crowd of joyful splashing. There is no course that can teach you how to de-escalate an irate pool patron who has just been told that they can’t hold their naked, cherubic infant in the pool. My manager, Rob, recognized my inexperience and took me under his tutelage. I soon discovered that Rob was the Smartest Person Who Ever Lived. Before my first time in a lifeguard chair, he advised “Drowning is quiet, don’t expect someone to yell for help.” He offered this other pearl of wisdom:
If you see a group of kids having fun, one of them is breaking a rule.
This has proven to be tautological.
This was Rob’s sixth summer at the pool, his last before starting medical school. Except for Dr. Solomon (my pediatrician) and maybe one of my friends’ dads, he was the first person I ever met who attended medical school. He had finished college, owned his own car, and had a very pretty girlfriend who dressed like a stylish teacher and smelled like very nice shampoo. Rob took me under his water wings, teaching about topics that went beyond the pool. Did he sense a kindred medical spirit because I was the only one who expressed interest in his anatomy coloring book? Or maybe because I let him use the awesome bench and free weights that I had in my basement? Regardless of why I was chosen, I sat enraptured at our lifeguard table on the pool deck as Rob held forth on a great many topics, from calculus (“It’s all about rates of change.”) to open-water kayaking. I was there for all of it.
Pool Chemistry 101 – Vats of Chlorine
As the pool operator, Rob had the keys to the pump room. This is where the complex pool chemistry was managed. Pump rooms are dingy, windowless affairs with poor air circulation, a chemical odor, and a constantly buzzing motor. I had just taken high school chemistry, but I knew nothing of the dark arts of pool disinfection and filtration. It can be helpful to have an extra set of arms to turn valves and flip switches and Rob found me to be an eager pupil and bucket carrier. The operator aspires to keep the pool water clean and comfortable. That is to say, the water must paradoxically be completely lethal to microbes, while simultaneously 100% healthy for swimmers.
From the outside, a swimming pool seems like a concrete pit filled with water, but there is so much more to it. There is also urine – we’ll get to that. The deceptively tranquil water is actually constantly moving; still water is dirty water. The 200,000 gallons at Apple Ridge Pool were pulled by 500-1000 gallon/minute electric pumps. Big, floating stuff, like hairballs, band-aids, and bloated frog corpses were caught by filter baskets at the water surface, which the lifeguards emptied by hand. Smaller debris was pulled through thick pipes into Volkswagen Beetle-sized filters filled with special sand.
Mechanical systems remove particulate matter, but chemistry is needed to kill germs. In our pool, like most in the US, we added chlorine for disinfection. (Bromide-based disinfectants are frequently used in hot tubs because of their heat stability.) To be precise, our pump room did not contain pure chlorine, but rather 100-gallon cylindrical vats containing a 12% aqueous solution of sodium hypochlorite (NaOCl). However, everyone referred to this viscous, translucent green solution as “chlorine.” (In six summers working at the pool, I never heard a single person use the term “sodium hypochlorite”) Sodium hypochlorite dissociates into sodium and hypochlorite (OCl-) ions, resulting in an equilibrium between hypochlorite and its conjugate acid, hypochlorous acid (HOCl).
NaOCl + H2O <-> OCl– + OH–
OCl– + H+ <-> HOCl
Other pool systems use tanks of chlorine gas (Cl2), which will also generate the HOCl molecule in water.
Cl2 + H2O → HOCl + H+ +Cl–
Negatively charged hypochlorite, which is repelled by bacterial cell walls, is less lethal to germs than its electrically neutral counterpart. Hypochlorous acid, in contrast, passes through the bacteria’s outer defense, wreaking internal havoc on those devious little e. coli that want to make your splashy swimming pool into a diarrheal disease delivery mechanism. We cannot yet turn tin into gold, but converting a fetid pool into swimmable or drinkable water is a miracle of alchemy. It is because of chlorine that you can drink public water and not have shigella coming out of your bottom — thank you chlorine!
A pump smaller than a shoebox pulled the liquid chlorine solution up into a plastic tube and injected it into the pool water line. For the most part, we were supposed to leave the vat alone. But, if the tube jammed, or wasn’t pulling NaOCl, Rob showed me how to troubleshoot the pump by unkinking the tube or running water through it. I carried this operation out with care because the concentrated NaOCl would burn a hole in my clothing if I didn’t immediately wash it. (This was a problem because the company-approved lifeguard shorts cost about twenty dollars and we only made about seven per hour. If chlorine ruined my uniform, my next 3 hours of work were just paying off my shorts.)
In accordance with Montgomery County Health Department regulations, we maintained the free chlorine concentration between 1 and 3 ppm, which is enough to kill bacteria but not irritating to human skin and eyes. (“Free” chlorine, the sum of HOCl and OCl– , is so-named because it is unbound, free to destroy bacteria.) We had a test kit, kept in a blue plastic box, with droppers full of mysterious liquids that caused colorimetric reactions with pool water. After adding the drops, a change from clear to pink indicated that the chlorine concentration was good. Nowadays, these millennial lifeguards probably have flux capacitors that measure the level of disinfectant, but we did colorimetry and we liked it.
As the proton concentration decreases, the equilibrium shifts from lethal hypochlorous acid to lazy OCl–. The pH is just an inverse measure of proton concentration, so hypochlorous acid loses its killing potential at high pH. High pH aso causes minerals like calcium to precipitate out, making your beautiful pool into a murky lagoon. This “hard” water isn’t unsafe, and perhaps provides a mineral spring ambience, but is not as aesthetically pleasing as a diamond clear pool.
The lower the pH, the more disinfecting HOCl that you have, but a pH of 7.4 is the sweet spot – sufficient to allow chlorine killing power but still comfortable for your eyes. (To an extent. When pH is less than 4, the HOCl shifts back to Cl2, but if you would have to commit serious managerial malpractice to get your pool pH that low.) Unfortunately, sodium hypochlorite raises pH, so our pool pH tended to creep up, making the water irritating to bathers and rendering the chlorine ineffective. We periodically had to add concentrated 32% hydrochloric acid solution to bring the pH back down into range. (Pools using gaseous Cl2 have the opposite problem. Gaseous chlorine lowers the pH, requiring periodic addition of sodium bicarbonate aka baking soda.)
The acid was stored in 55-gallon black drums in our pump room. Although the liquid appeared clear like water, it had an acrid odor that irritated my eyes and burned my nose hairs when I opened the drum. To add it to the pool, we transferred it from the black drum to a five gallon bucket using a plastic siphon drum pump, and then dumped the bucket into the pool. We were supposed to have water in the gallon bucket first – always add acid to water! – but in practice that made the bucket heavy. A heavy bucket banged and scraped on the side of my calf as I lugged it across the concrete pool deck, so I preferred to have a bucket of acid without added water. I was always very careful when carrying out this operation, because I suspected the acid would burn skin. (One of my fellow lifeguards had reportedly melted a He-Man action figure with the hydrochloric acid solution, and that story was good enough for me.) Many pools nowadays have automatic feed systems that directly add the chemical, so dumb teenagers don’t need to haul around buckets of acid, and this seems reasonable beyond belief. We had buckets.
When the pool has a “chlorine” level of 1 to 3 ppm and a pH of 7.4, it has reached a state of complete perfection and clarity. In the morning, before the swimmers arrived, I would park my deck chair right on the edge. If the air was still, the pool looked like a single solid gemstone and, except for the slight bluish crystalline hue, the water almost wasn’t there at all. This Eden-like state could not last, because the pool was made for the swimmers. As Rob used to say, “The pool would be great if not for all the swimmers.” As I will explain to you, Rob’s words are scientifically, objectively true.
Nothing Gold Can Stay
Two factors deplete chlorine in an outdoor swimming pool. The first is the sun. Visible sunlight (wavelength 330 nm) and ultraviolet light (255 nm) degrade hypochlorite and hypochlorous acid. Although formed 93 million miles away, the radiant energy formed by hydrogen fusion will smash your hypochlorous acid to bits.1
Energy From the Sun Breaks Down Disinfectant HOCl2
2OCl– + sun → 2Cl– + O2 (Faster)
2HOCl + sun → 2HCl– + O2 (Slower)
This is a manageable problem. The pool operator can add cyanuric acid, an inert, non-caustic triazine to protect the chlorine. (A triazine is a ring-like organic compound with three nitrogens and another element, usually carbon.)
Cyanuric acid acts as sunscreen for the chlorine. The nitrogen atoms in the triazine ring form weak links with OCl–, protecting it from sunlight. At our pool’s goal level (30-50 ppm) the chemical is harmless. When the concentration gets too high, cyanuric acid gets a little too good at binding chlorine and the killing power of your disinfectant decreases. In fact, the CDC guidelines recommending keeping levels of cyanuric acid less than 15 ppm in the event of a “fecal incident.” WHERE ARE YOU GOING?
This segues nicely into the cause of chlorine depletion, person-made organic nitrogen compounds. The three main sources are soil, sweat, and urine. Mostly urine. According to one estimate, the pool contains 30 to 80 mL of urine for each swimmer. Organic nitrogen compounds react with chlorine to produce byproducts that are irritating to people and weak as disinfectants. The most important of these chemical byproducts, chloramines, cause that recognizable fishy pool smell and eye irritation. Monochloramine is the most common chloramine at a typical pool pH, but at a lower pH and higher ammonia concentrations, dichloramine and trichloramine are produced. Other disinfection byproducts, such as trihalomethanes (including chloroform) and haloacetic acids aren’t present in sufficient quantities to cause discomfort, but have been suggested by some to cause detrimental long-term health effects.
Monochloramine Production from Ammonia and HOCl
NH3 (aq) + HOCl– → NH2Cl + H2O
- Hypochlorous acid (formed when we add sodium hypochlorite to water) kills germs
- Sodium hypochlorite (our source of hypochlorous acid) tends to raise pH, which makes disinfection less effective
- We add acid to drop pH down to a good level for disinfection
- To avoid irritation of your beautiful, delicate eyeballs, we don’t like pH to go below 7.4
- That damn sun wants to destroy hypochlorous acid, but we can protect it with cyanuric acid
- When cyanuric acid level is too high, the chlorine won’t work well
- Organic nitrogen compounds turn hypochlorous acid into weaker, smellier, more irritating compounds- chloramines
- The “high chlorine” isn’t bothering your eyes. That chlorine smell is from chloramines, and it’s your fault, you filthy animal . . .
From the Lifeguard Chair to the Bedside
You have now learned enough chemistry to get through a summer of lolling around the pool in a floppy hat telling kids not to run. When I was 17, monitoring pool pH, pulling kids out of the water, and counting down the minutes to my next lunch break, I never thought I would need to ponder the chemistry of chlorine compounds with acids and ammonia again. As it happens, there are real-world clinical ramifications of this pool chemistry. A review of the 2017 National Poisoning Data System annual report reveals 4039 chlorine exposures, including one death.
Don’t Mix Bleach and Ammonia
Adjacent to your washing machine, you have a bottle of bleach (sodium hypochlorite) with a capitalized warning, “DO NOT MIX WITH OTHER CLEANING PRODUCTS. MAY RELEASE DANGEROUS GASES.” They nailed it.
Why would anyone make the mistake of mixing bleach with ammonia? In many cases, this is a simple mistake caused by adding one cleaning solution into a cleaning bucket that unknowingly had residual liquid. Others mix them intentionally, assuming that combining two products will result in one super cleaner that whitens whites and clears windows. Every now, and then, a well-intentioned person tries to clean a septic system with bleach, not contemplating the effect of combining hypochlorite with urine. In all the above cases, the result is the same chemical process that takes place when HOCl reacts with organic nitrogen compounds in the pool – chloramine formation.
NH3 (aq) + HOCl– → NH2Cl + H2O
Low concentrations of chloramines dissolved in your pool, bother your eyes, and create an odor. High levels of the chemical form a potent vapor. Chloramines are probably the most frequent irritant gas exposure seen in the emergency department. The National Poisoning Data System received 2115 reports of chloramine exposures in 2017. Although none were lethal, four were considered “major” or potentially life-threatening.
Pulmonary irritants, like chloramine, damage tissue. Clinical severity of that damage is determined by the concentration of exposure, duration of exposure, and the anatomic location of the exposure. In practice, the water solubility of the compound is the key determinant of all of these factors. More water soluble compounds, like chloramines, rapidly cause upper airway symptoms and eye irritation. Thankfully, these symptoms serve as warning signs that signal people to leave the area, so patients don’t usually have severe lung injury. The highly water soluble irritant ammonia, for example, may cause Acute Respiratory Distress Syndrome (ARDS) in as few as 2 minutes, but this is rarely seen because no one sticks around the noxious vapor for that long.3 This is not true of pulmonary irritants with low water solubility. Because these lack warning properties people can inhale enough of them to cause lung injury. Since chloramines have good warning properties they won’t cause severe lung injury unless patients are in closed spaces or immobile.
Don’t Mix Bleach with Acid, Either
Mixing bleach with acid will cause an even more consequential problem: chlorine gas.
HCl + HOCl → Cl2 + H20
Poet and WWI soldier Wilfred Owen described a gas attack in “Dulce et Decorum Est.”
Dim through the misty panes and thick green light,
As under a green sea, I saw him drowning.
In all my dreams before my helpless sight,
He plunges at me, guttering, choking, drowning.
The American Chemistry Council reports that chlorine gas is released at swimming pools about a dozen times per year. In San Jose, a worker inadvertently mixed sodium hypochlorite and hydrochloric acid in the pump room, resulting around 30 relatively minor exposures, ranging from minimal symptoms to vomiting and coughing. In other cases, malfunctioning equipment is to blame. When sodium hypochlorite and acid feeder pumps continue to add chemicals to the pool water line when the pump is turned off, a growing bubble of chlorine gas forms in the unmoving column of water. When the pump restarts, the accumulated gas is forced into the pool and rises to the surface.
Many experts consider a mass casualty incident with industrial agents like chlorine – either unintentional or not – to be more likely than a terrorist attack with sarin or other nerve agent.
According to one model, a crash of a 10 ton chlorine tanker truck in an urban area could contaminate 2 square kilometers and result in 5000 lethally-exposed people.5
Clinical Effects of Chlorine Gas
Owen’s description of choking, drowning in a green sea is scientifically accurate. With an atomic number (17) greater than oxygen (8) and nitrogen (7), heavier-than-air chlorine gas forms a greenish ground-level cloud that tends to hang unless taken away by wind. Chlorine penetrates deeper into the lungs than water soluble gases, causing pulmonary edema – “drowning” that is medically described as acute respiratory distress syndrome.
These pulmonary effects may be persistent in some patients. Acute exposure to pulmonary irritant can cause a chronic condition- reactive airways dysfunction syndrome (RADS). Patients develop an acquired asthma-like condition with intermittent wheezing and bronchospasm. Experts consider RADS to be a distinct entity from occupational asthma because the microscopic appearance of bronchioles is different and the cause is a chemical- not allergic-sensitizer.6
Management of Chlorine Exposure
The management of chlorine exposure is straightforward and basically supportive. Check a chest x-ray for pulmonary edema, administer humidified oxygen and bronchodilators. Other therapies have been investigated with mixed results. A prospective study of nebulized sodium bicarbonate administration to patients with chlorine exposure did not find clinically significant improvement, although the intervention group had better FEV1 at 120 and 240 minutes.7 (Recipe for nebulized bicarbonate: Add 1 mL of sodium bicarbonate (7.5% or 8.4%) to 3 mL sterile water for nebulization.) Other interventions for chlorine exposure, such as corticosteroids, ascorbic acid/deferoxamine, dimethylthiourea, have been used without clear benefit.8
So Summer Ends. . .
Autumn came, the pool closed, and our staff went our separate ways. I went into my senior year of high school, most of the others scattered to various colleges, and Rob left for medical school. In total, I spent five more summers working at swimming pools. Although most of my college pre-med classmates took resume-building internships, I eschewed the air-conditioning for a few more years of chlorine and humidity. During the last three seasons I was a licensed pool operator and manager, maintaining the pool chemistry and teaching the craft to junior lifeguards.
I can’t say that I became a physician because of that first summer experience. However, it wasn’t until working with Rob, talking with a young adult that was actually going to medical school, that I considered medicine as a career that could even be a possibility for me. Last I heard, he was planning on a career in anesthesiology. Presumably, he had mastered the pumps and tubes of the pool filtration room and wanted to move up to a bigger challenge without completely leaving pumps and tubes and chemicals behind.
Apple Ridge Pool, now renamed after our old boss’s boss, has been completely renovated. They put in a lazy river and ground spray system that I would have no idea how to operate. As I type this, it is 12p and the pool is opening. Doubtlessly, a horde of children are pressed up against the gates waiting to splash and urinate.
The irony was, of course, that working at a swimming pool prepared me for my current vocation better than anything else I did. In the emergency department, as in lifeguarding, I work with a small team of trained colleagues. I placate unhappy customers. I order food. I save lives. I deal with problems of chemistry.
We just need an ice cream truck.
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- 3.Sjöblom E, Höjer J, Kulling P, Stauffer K, Suneson A, Ludwigs U. A placebo-controlled experimental study of steroid inhalation therapy in ammonia-induced lung injury. J Toxicol Clin Toxicol. 1999;37(1):59-67. https://www.ncbi.nlm.nih.gov/pubmed/10078161.
- 4.Nelson L, Odujebe O. Simple Asphyxiants and Pulmonary Irritants. In: Nelson L, Howland M, Lewin N, Smith S, Goldfrank L, Hoffman R, eds. Goldfrank’s Toxicologic Emergencies. 11th ed. New York: McGraw Hill; 2019:1651-1662.
- 5.Brzozowska L. Computer simulation of impacts of a chlorine tanker truck accident. Transportation Research Part D: Transport and Environment. March 2016:107-122. doi:10.1016/j.trd.2015.12.001
- 6.Brooks S, Hammad Y, Richards I, Giovinco-Barbas J, Jenkins K. The spectrum of irritant-induced asthma: sudden and not-so-sudden onset and the role of allergy. Chest. 1998;113(1):42-49. https://www.ncbi.nlm.nih.gov/pubmed/9440566.
- 7.Aslan S, Kandiş H, Akgun M, Cakir Z, Inandi T, Görgüner M. The effect of nebulized NaHCO3 treatment on “RADS” due to chlorine gas inhalation. Inhal Toxicol. 2006;18(11):895-900. https://www.ncbi.nlm.nih.gov/pubmed/16864407.
- 8.Huynh T, Despréaux T, Loeb T, Salomon J, Mégarbane B, Descatha A. Emergency management of chlorine gas exposure – a systematic review. Clin Toxicol (Phila). 2019;57(2):77-98. https://www.ncbi.nlm.nih.gov/pubmed/30672349.