Dear Mark: Swimming Pool Chemicals, Washing Veggies, and Carb Blockers

Who doesn’t like a lovely day at the pool? Unless you can’t swim, there’s no reason not to love the cool water, the bright sun, the ping pong (every swimming club worth a dime has a ping pong table, or several of them), the face dunking, the high dive, and the chicken fights. But what if something sinister churned within the depths of the chlorinated water? What if by entering that pool you were risking life, limb, and the pristine alabaster of your eyeball? In today’s edition of Dear Mark, I’ve gone back to the roundup format. I begin with the question of swimming pool chemical safety, follow with a query about washing vegetables, and I finish the post with a short section on carb blocking agents. Sound good?

Let’s go:

Dear Mark,

I was wondering about possible negative effects of pool water. I enjoy a good sprint workout in the pool, especially when traveling and staying in a hotel. Could the chlorine or other chemicals be harmful since they do sometimes make me itch a little afterwards and burns my eyes (especially if I open them underwater)?

Thanks and Grok on,


I hate to be the bearer of potentially bad news, but there’s probably something to this. Most pools use chlorine as a disinfectant, to keep the water clear of bacteria and other microbes, and it’s darn good at that. Reason? Chlorine, in its pure form, is toxic. The chlorine in the pool is obviously diluted, so it’s not going to burn or kill you outright, nor are you a microbe, but toxicity concerns remain. Your first clues that it might be doing something untoward, of course, are the burning eyes and itching skin. That’s pretty normal, albeit disconcerting. As a kid, I used to get red, burning eyes when I’d spend the day at the pool. Nowadays, I think back to that and wonder…

Anyway, red eyes clear up and itchiness subsides, but could other problems be lurking beneath the surface? Maybe. Chlorine reacts with other substances, including bodily fluids and various organic matter, to form disinfection byproducts (DBPs), which may have novel – and unwanted – health effects. Let’s take a look at some evidence:

  • When chlorinated pool water meets dimethylamine (found in urine and sweat), nitrosamine carcinogens (the same type of compound that forms when we overcook bacon) form, and appear in pools at concentrations up to 500-fold higher than drinking water. Though it’s unclear whether or not these particular nitrosamines are absorbed by pool users, some nitrosamines are absorbed through (rat) skin. Why should you care? Well, nitrosamines are used to induce bladder cancer in rodents, and chlorinated pool and bathing water usage have been linked to bladder cancer in humans (though it’s just observational).
  • Chloramine, another DBP, has been linked to asthma in pool workers and elite swimmers.
  • A recent study found over 100 chemical byproducts in swimming pools, many of them toxic. Before and after 40 minutes of swimming laps in such a pool, healthy subjects’ biomarkers were tracked and recorded. One marker suggested increased lung permeability and inflammation, while another marker indicated a kind of DNA damage that, if unchecked, might lead to cancer. Subjects had also accumulated elevated levels of four of the most common DBPs after 40 minutes in the pool.

The good news is that you’re probably okay. Problems may arise when we absorb and uptake these DBPs (like chloroform) via inhalation, dermal absorption, and the ingestion of affected water on a regular basis. The populations that seem to suffer most from pool-related maladies are the ones who spend significant amounts of time at, in and around the pool – competitive swimmers (with their infamously long daily workouts), lifeguards, and other pool workers – and it doesn’t sound like you’re living in the water. If you stick to short, intense sprints, performed only when you have access to a pool on business or vacation, I wouldn’t worry.

Hello Mark,

Can I wash my veggies with dish washing solutions? Or must I use special vegetable washing solutions?

Thank you for your time,


Actually, you don’t have to use either. Tap water will work just as well. But don’t take my word for it. Check out the results of this study that explored this exact question. They used tap water, Palmolive, and four different vegetable washes to process unwashed, pesticide-laden produce and found no differences in pesticide residues when all was said and done. Luckily, washing the produce – whatever the solution used – took care of most of the surface pesticide residues (not all of them, though, not to mention the pesticides that are integrated within the produce).

So, yes, dish washing solution will work just as well as special vegetable washing solution, but so what? Water does the job, too.

Hi Mark,

Whats the deal the Carb-inhibitor/blocker pills, do they work? Are they safe? If so, which ones do you recommend? Thanks.


Carb blockers use an extract of the white kidney bean that inhibits alpha-amylase, a digestive enzyme that breaks down starch. Without alpha-amylase doing its work, we can’t effectively digest starches, and they pass through to the small bowel to be fermented by gut flora. Sounds great, right? You get to eat carbs and you don’t digest them. They don’t turn into glucose, they don’t get absorbed, and insulin stays low.

I kid, but actually, a study shows that this is pretty much what happens. On the first day, subjects ate 50 grams of rice starch. On the second day, they were administered an amylase-inhibitor that inhibited 95% of amylase activity and fed another 50 grams of rice starch. Postprandial (post-meal) delivery of carbohydrate to the small bowel was increased after eating the carb blocker, meaning less was absorbed. Blood glucose spike was reduced by 85%. Insulin was “abolished.” What’s not to like?

I’m a little suspicious of something that “blocks” a normal physiological function. Just because I think we should reduce our reliance on carbohydrates as energy sources doesn’t mean I no longer value our natural, inherent ability to digest them. I’m also suspicious of shuttling all those fermentable carbohydrates to our gut flora. Giving some soluble prebiotic fiber? Cool, that’s great and we evolved eating fibrous vegetable sources, so our “normal” gut flora is likely used to it. But it sounds like providing a massive dose of something like sweet potato starch to our eager gut flora is a potential recipe for small intestine bacterial overgrowth (SIBO), which we definitely don’t want. There’s evidence that blocking amylase action indeed increases short chain fatty acid production by our gut flora, a marker for gut flora activity, but instead of absorbing the healthful fatty acids, those with impaired amylase activity excrete most of them. That tells me that maybe the gut flora are biting off more than they can chew, that maybe providing all that cheap starch to our small bowels is too much of a good thing.

But that’s just speculation off of a few related studies. We can’t know for sure, of course. Still, if you want to block carbs, just don’t eat so many of them. That’s certainly safer than messing with a vital, inherent part of our physiology, don’t you think?

Thanks for reading, folks. Let me know what you think in the comment board. Grok on!

TAGS:  dear mark, toxins

About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

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82 thoughts on “Dear Mark: Swimming Pool Chemicals, Washing Veggies, and Carb Blockers”

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  1. I think that hotels these days use something other than chlorine to keep their pools clean. But that could just be something I read somewhere that I took as fact.

  2. I’ve noticed that the pool water tastes somewhat salty instead of the chlorine bleachy taste I remember from my childhood. Does anyone know what’s changed?

      1. It’s just a matter of the operators of the pool changing the source of the chlorine. Salt systems are becoming increasingly popular and they get the chlorine from NaCl (sodium chloride, aka table salt). The salty taste is from the sodium. So it’s generally thought to be safer, less concerning than traditional sources of chlorine which are more chemical-y, that is to say more toxic smelling and such.

    1. The salty taste suggests that the pool is treated with bromine as opposed to chlorine. As a former competitive swimmer for a long long time, we used to take great joy in swimming in a bromine-treated pool, just because it gave a nice change of taste. (Swimmers are weird).

    2. Well many pools are “saltwater” pools instead of chlorine now. Otherwise… ewww.

    3. My gym, at a minimum, uses a mixture of salts to keep their pools clean. Zero chlorine, and they (smartly) use this as an advertising perk.

      Now if only I could convince them to carry primal fuel shakes instead of fruit smoothies…

    4. That’s because newer saltwater generators are being used for pool disinfection. NaCl is hit with an electric current which breaks it down to Sodium and Chlorine. The chlorine is what ultimately does the disinfection of the pool water.

  3. Yes, many pools seem to have switched over to a saltwater system. I think it’s supposed to be healthier.

      1. Wasn’t Mark advocating for fecal transplants? Here’s our chance…

        Or you could just use a hardboiled egg. Floats = salt water.

      2. I think that you can get snickers “floaters” when the individual eats a lot of fat too. Just saying.. I guarded salt and chlorine pools during swim lessons. saw floaters in both… I don’t suggested tasting the water. if you knew how much organic matter “nice word for poo” was floating there you would be sick. about 25lbs (i think i remember) in a 6 lane 25yd pool. yummm. thanks for the article.

    1. After reading this wonderful blog for months now I can;t resist finally posting something – Mark, keep the good stuff coming!

      Saltwater in a pool is not necessarily healthier in and of itself, but it is a more reliable way of dosing a pool with chlorine. The way it works is there is an electrolysis cell that reduces the aqueous chloride ion back to the reactive form – so it can do its disinfection. The system monitors the free chlorine level and turns itself on and off as needed. You then only need to add salt to the water instead of handling hazardous hypochlorite compounds.

      Systems like this have been used in domestic and public pools in Australia for decades. They are safer and simpler to operate, and many people (including myself, when I was growing up there) report less skin and eye irritation.

      Of course, if you have access to the *real* saltwater pool- the ocean – it can’t beat, and is minimal maintenance too.

      1. Heh. Not so much in northern California. The water is 60F/15C on a good day and if the beaches aren’t just rock faces, the wind will flay you with a sandblasting. You can put on a wetsuit, but there’s still the rip currents and undertows, and the occasional Great White. Our ocean is mostly just for looking at. 😉

  4. What about washing potentially harmful bacteria off of organic produce? I don’t worry about pesticides because I buy organic, but I still wash my produce…is that silly? And wouldn’t soap work better for that? Any good studies done there?

  5. Just returned from my morning mile in the pool to find this article waiting for me. Maybe I’ll consider the pool my 20%.

  6. I used to think it was great to go out to my garden and pick a few lettuce leaves and a couple of tomatoes so that I could enjoy a tiny salad right there under the sunshine. Since I grew it, I knew it was organic and clean. Then I found bird poop on some leaves and saw my dog hike his leg and pee on some plants. Now I wash everything!

    1. hahaha That would certainly make me thing twice about eating stuff straight from my garnden, too.

  7. There is a lot of absorption of Chlorine through showering everyday. Not shower like GROK? Heavens no! We purchased an inexpensive (about $110.00 at SAM’s Club) simple self-cleaning whole house water filter (only removes 30 microns and larger) – but works well enough to remove the minimal chlorine our town adds to its water. Thankfully, our town does not Flouridate its water. Otherwise we’d have to revert to a more expensive installation and maintenance whole house water filter system.

  8. “If you stick to short, intense sprints, performed only when you have access to a pool on business or vacation, I wouldn’t worry.”

    Agreed. So much of what we do has pros and cons. We have to take the good with the bad. Of course, when the bad outweighs the good, it is time to pursue other endeavours.

  9. According to Expert Foods Expert Foods, the ThickenThin not/Starch thickener product s not available. And according to NETrition: ThickenThin not/Starch thickener (Manufacturer out of business.)
    NETrition also offers a similar product: Dixie Diner Carb Counters Thick It Up Low Carb Thickener 6 oz. 227 g, but also ists it as: Not available for sale.

  10. Damn I used to spend a lot of time in the pool when I was a kid, I’m probably living on borrowed time

  11. My mistake, the Dixie product IS available!
    Ingredients: Locust bean (and/or tara), guar, acacia, xanthan gums.
    Recommended Use: Add 1/2 teaspoon per cup of liquids such as soups, gravies, sauces and fruit fillings. Whisk or stir into liquid, heat until reaching the desired thickness. Liquids will continue to thicken as they cool.

  12. My only expertise on this subject (pool water) comes from the fact that I swim on average 5 or 6 times a week for an hour and a half each time, and I am a regular reader of the US Masters Swimming forums. The subject is frequently discussed there, if anyone cares to delve further. Most of the concern these days seems to revolve around chloramine, which is concentrated just above the surface of the water. Apparently, chloramine’s effects can be ameliorated by an adequate exhaust system. Unfortunately, many pools are more concerned about conserving energy and are loathe to exhaust heated air from the pool room. So it’s important to pick a pool that’s adequately ventilated.
    Likewise, swimmers should look for pools that are adequately chlorinated — neither too much nor too little. Lower water temperatures require less chlorine (and are more pleasant to train in). Alternatives are pools that are disinfected with bromine gas (side-effects unknown) or that use salt water (I love salt water pools!) And in summer, there’s always open-water swimming.
    I personally experience no problems from swimming in a chlorinated pool, but I always shower thoroughly afterward. I think the physical benefit to be gained from swimming far outweighs the potential danger from chlorine. Swimming is a low-impact sport that can be sustained for life. Who ever heard of a masters footballer?
    P.S.: At my last meet I witnessed a 93-year-old woman swim a 100-yard IM (which for the uninitiated, incorporates all 4 strokes, including butterfly, the most strenuous). I assumed she had been a competitive swimmer all her life, but l later learned she had taken up butterfly only five years previously. She wasn’t the fastest swimmer in the race, but by god, she received a standing ovation from the crowd when she touched the wall at the finish!

    1. That really heavy “chlorine smell” in most indoor pools is actually chloramine, and it usually indicates *inadequate* chlorination, but inadequate ventilation will do it too.

      Swim in an outdoor pool if at all possible – the sun on your skin doesn’t hurt either

    2. the chloramines that form about 6-18 inches off the top of the water are seriously bad for our lungs. Outdoor salt pools would be the best. or a very well ventilated pool. if you think about it, while doing laps this is right where you are drawing your deepest breaths. I agree with you masters is a great resource…

  13. I typically soak the vegetables in a mild vinegar solution. Also, I’ve heard bleach can be used, all followed by a good rinse of course.

  14. Even for non-swimmers there is a concern of chlorine exposure through daily showering/bathing. Unless you filer your water or live in a city that does not add chlorine to the municipal water supply, you are absorbing chlorine (and breathing chlorine gas) while showering. If you are not in a situation to filter all the water entering your home, a shower (or bath) filter can lower your exposure and can help skin issues as well.

    1. Or, you can have a well, and not chlorinate your own water. I hope I never have to go back to municipal water on a regular basis.

  15. Much of the eye irritation people experience comes from pool water with a ph level that is off the recommended range, which impacts the ph level in your eyes. I won’t say the chlorine isn’t in play here as well, but everything I’ve heard was it was the ph.

    I’ve seen lots of pool equipment go up before it should b/c the homeowner ran their pool with really low ph for an extended time.

    “Saltwater” pools still use chlorine. Note the “Cl” part of the scientific description of salt (NaCl). The big difference is instead of adding cholrine tablets to a skimmer or chlorinator like most pools, people with a “saltwater” pool add salt and a device is added near the pool’s filter to remove the chlorine from the salt (NaCl).

  16. I use a solution of grapefruit seed extract mixed with water and spray it on my veggies and rinse ’em off!

  17. When I saw the title I thought this would be a post how increased presence of bromine, chlorine and fluorides in our bodies blocks iodine and interferes with thyroid function. I have read a lot about it was hoping for Mark’s thoughts.
    Now I have all these other things to worry about 😉

    1. That’s interesting. Could explain the monthly psychosis beyond ye olde carb-loading when I swam hard three/four times per week.
      Thanks for the direction Nicola.

    2. This really is interesting!

      I have impaired thyroid functioning and I can’t go swimming at all in a normal pool. When I’m in the water, my skin itches like hell and I can hardly open my eyes (although I usually don’t have any skin issues at all). The following days, I feel really really sick. I have dry cough, my eyes water, my nose runs. I feel like the energy has been drained out of my entire body and I can’t think properly.

      Swimming outdoors is better, but still far from perfect. Swimming in the sea, however, works well.

  18. The absolute best carb blocker in existence is a big human brain directing the arm and hand to not stuff the face with carbs.

  19. When i was in Australia, a lot of my friends and some commercial pools used salt water rather than chlorine. I wonder how effective it is? Certainly less toxic and burny.

    1. I think most of our saltwater pools here use some chlorine as well. I think the salt just means you don’t need to use as much chlorine.
      I could be wrong

  20. I soak my vegetables in lead-free gasoline for 10 minutes. Then I light it. A very stylish flambé. And all microbes are dead forever.

  21. Chlorine in pool water is one thing, but chlorine in your household water is a reality for most people with municipal (city) water. The dangers of chlorine in your household water are the same as Mark has outlined, but even worse because of your constant exposure.

    You may not have considered these sources of chlorine gas and associated disinfection byproducts (DBPs):

    -Dishwashers – Most people wash their dishes using hot water, and many even use the ‘hot’ dry cycle as well. This generates steam that can easily be released into the air you breathe.
    -Washing Machines – Using cold water to wash your clothes certainly helps minimize vapor release. But washing your clothes in chlorinated water can embed the chemical into the clothes you wear, potentially leading to rashes and other irritations.
    -Toilets – The flushing action of a toilet potentially releases chlorine vapors into your indoor environment. Regardless of the flushing, unfiltered water just sitting in the bowl has the potential to release chemical vapors.

    A good whole house filtration system is your best solution because these vapor sources would all need their own filters and their own replacement filters. By the time you add up the expense it is easily seen that a whole house system is the best choice.

    I have for years used the most reasonably priced system. It does the best job for the price – The V-Series Whole House Water Filtration Systems from Vitasalus. They are highly-specialized systems designed to remove up to 99.9% chlorine, VOCs, THMs, atrazine, benzene, pesticides, insecticides, tastes, odors and hundreds of other potential contaminants and harmful chemicals from the water entering your home.


    Learn more:

  22. I don’t wear sunscreen or any other chemical-laden products, so swimming in a soup of all that stuff plus the chlorine or whatever else they’re using, combined with the fact that more often than not the levels of the stuff aren’t properly maintained – sadly, all this just eeks me out too much. I’d love to have my own pool though – I’m sure there’s gotta be a better way to keep it clean..

  23. Having grown up next to the ocean, and having loved rough water swimming with a passion all my life, I’m not sure chlorinated pools are really that worrisome when compared with getting hepatitis and parasites and other fun things from raw sewage run off in the ocean.

    It’s a wash for me, when sharks and jellyfish are removed from the equation.

  24. I almost always have issues with itchy skin after a good swim. It was really bad last year when I was hitting the pool 3-4 days/week. I tried to cut back to just once a week this year. I love swimming more than any other exercise so this has been hard on me!

  25. My husband used to tease me that I was ‘off to do my water aerobics in the sewer’ — but it concerned/concerns me that the (local Y) exercise pool is conjoined with the lap lanes AND the kiddie pool. I do NOT believe that swim diapers are sufficient “control” of (potential)contamination. It’s dismaying. I try to keep my face out of the water, and shower thoroughly after — but now it’s the outgassing just above the water surface?! Sheesh!

  26. I thought I’d read at some point that keeping the Ph in check on your pool water using plain ol Baking Soda eliminated the need for Chlorination, since algae etc wouldn’t grow in a proper Ph environment. Any pool peeps know if this was just hog wash? I’m guessing since all the other comments no one brought this up I may just be inventing B.S. (BarbraStreisand) in my warped little mind again.

  27. When I was a kid, I took swimming lessons for several summers at our local playground pool and used to just love to get in the water – I’d go every day I could when the pool was open during the summer. Little did I know—- (Anybody here remember when public pools were closed because the polio epidemic was thought to have been transmitted in public pools? – And yes, polio vaccines quelled that scare somewhat.)

    Now that I know a lot better, there’s no way – and I DO mean NO way — I’m going into a public pool of any sort. I don’t care how much chlorine, salt water or what ever it is made up of – to me pools are nothing but giant toilets and/or bathtubs comprised of water used for soaking the private parts of various bodies — previously cleaned or otherwise.

    At least in the ocean we have the benefit of all the nasties from the various “unclean” parts of our fellow bathers to be assimilated in untold gallons of water, and hopefully diluted to the point of dissimilation. (Chemicals and other disgusting things dumped from industrial sources notwithstanding—)

    1. Yes, I’m old enough to remember about polio and it’s association with swimming. My older brother had polio (not severe) and Mother said he got it when he went to a local swimming hole.

      And speaking of polio and vaccines, have you read the book “The Virus and the Vaccine” by Debbie Bookchin?

    2. Where do you think fish, dolphins, turtles, crabs, shrimp, etc. etc. do their business? Did you know there are more bacteria in a gallon of sea water than there are humans living on earth?

      1. Saltwater can kill pathogens…

        Clorine and Bromine are both halides that compete with iodine for receptor sites. Just say’n.

  28. I can’t recall the last chlorinated pool I swam in, nearly every pool in my town is salt these days… Australia… I thought this was common?

    Spa’s though… outdoor hot tubs… woah the gas coming off those…. THAT really gets me feeling like Im sniffing chemicals… eeek

  29. All of a sudden I´m extra thankful for living in Iceland. It may be cold but our outdoor swimmingpools are relatively clean from chemichals (natural hot water may smell like rotten eggs but it does take care of some of the yuckier human-body-bacteria). The wind that blows almost incessantly takes care of the rest. 😉

  30. Showering in chlorinated city water is toxic because it is breathed in as well as absorbed by the skin. I understand the breathing of the vapor is the worst. I use a shower-head filter that can be purchased from any home store (Lowe’s, Home Depot)for under $19.00. It easily attaches to the shower head and removes the chlorine. You have to replace the filter insert every 6 months($10), but still a cheap fix.

    1. I use one of these, too. City water generally contains too many toxins. Best to use a filter system. It’s cheap.

  31. Funnily enough, we stayed at a hotel near Disneyland recently (just before Bat’s Day in the Fun Park) and I’m fairly certain the pool there was salt-water. I could taste it on my lips afterward and my hair didn’t get the sticky feeling it usually does with chlorine.

  32. I use a spray bottle with some white vinegar in it and spray my veggies, then run a light veggie scrubber on them. I used to also use hydrogen peroxide but I’ve gotten lazy.

    Why? It kills a lot of bacteria which can be on produce. Here’s some info on the research:

    Cheap, and non-toxic.

  33. There has to be a better way than just water to safely wash most pesticide residue off fruits and veggies, for those whom organic is not an option. Any more suggestions? Thanks!

  34. I used to get wicked nosebleeds as a kid from getting in the pool. It was hard to learn how to swim when you couldn’t put your face in the water. I still wont… I wonder if its something one might “grow out of.”

  35. What about ozone instead of chlorine in swimming pools? I think most of the swimming pools where I live use ozone instead of chlorine.

    1. Steven, Ozone is a strong oxidant and isn’t good to breathe in either. However it prolly is possible to get the ppm level where it would kill bacteria and not your lung cells.

  36. Don’t any pools use UV light to sterilize the water? Some reef aquarium enthusiasts do that to kill unwanted pathogens and parasites. Water is run thru a hollow bulb that emits concentrated UV radiation. Then all you’d need is a particle filter.

    It’s prolly available and I’d google it.. But since I can’t afford a pool atm I don’t really care. =)

  37. I swim twice a week for 90 minutes a day (180 minutes in all). Is it ok?. I have definitely tanned. and my skin has become a little sensitive. Is it alright?