Peer-Reviewed Scientific Research
We generally only look at peer-reviewed papers in respected scientific journals and even then we look at multiple papers giving greatest weight to surveys of papers that compile the entire landscape of information. We also look at who funded the researchers and any conflicts of interest. I've personally purchased well over a thousand dollars of these papers over the years since reading the abstracts rarely gives the complete picture of what is going on. We do this because reports from websites that are selling products are generally dubious since they have a financial incentive to deceive. It's not that they are all wrong, but we've found them to be very mixed -- some are completely incorrect, some are correct but intentionally leave out relevant information (i.e. are deceptive), and others are fairly reasonable.
Chlorine Absorption
So let's talk about the issues raised in the three links you provided, all of which have a financial incentive to be biased. First of all, chlorine is not absorbed very deeply into the skin let alone the body because chlorine is very reactive. It mostly reacts very quickly with the ammonia in your sweat within seconds to a minute to form monochloramine at which point it much more slowly reacts. Monochloramine is what is found in chloraminated drinking water and many municipal water utilities have switched from chlorine to monochloramine so that fewer disinfection by-proudcts are produced in the water distribution system. Some of the chlorine oxidizes chemicals in the skin and when it does so it becomes chloride (salt). Some chlorine reacts with other chemicals in the skin and becomes combined chlorine (CC) and it is some of these chlorinated organics that have dermal absorption and can enter the body and bloodstream. In fact, if chlorine itself mostly absorbed into the body, you wouldn't have the side effects of flakier skin or frizzier hair when exposed to higher active chlorine levels. When people do the experiment of dipping an arm into a small container of water with parts-per-million amounts of chlorine in it, the chlorine goes away and they claim the chlorine is absorbed, but that's simply not true and instead the chlorine has mostly reacted with the surface layers of your skin (mostly the stratum corneum layer of dead cells and the ammonia and creatinine in sweat).
Disinfection By-Products
The most notable category of the combined chlorines are the trihalomethanes (THMs) that I discussed before. The other main category are the haloacetic acids (HAA5 because there are 5 of them normally tracked in drinking water, though there are 9 total). There are also haloacetonitriles (HAN). These three are also found in pool water as disinfection by-products from chlorine reacting with various organics found in the water (i.e. not just on skin). Another main category of disinfection by-products are the inorganic chloramines: monochloramine, dichloramine and nitrogen trichloride. Whereas some of the THMs, HAA5 and HAN are of concern due to their potential to cause cancer, nitrogen trichloride is of concern due to irritating the airways. I should reiterate that the quantity of disinfection by-products found in pools is strongly correlated with bather load and almost all of the pool studies are with high bather-load commercial/public pools. Residential pools, especially outdoor ones using CYA, generally have much lower levels of disinfection by-products and just as importantly, the CYA significantly moderates chlorine's strength to significantly slow down the production of those by-products in skin.
Epidemiological Studies
So let's first look at one of the most comprehensive surveys of chlorinated water that was recently done -- namely the
Environmental Health Criteria 216 "Disinfectants and Disinfectant By-Products" document from the World Health Organization (WHO) from 2000 (updated somewhat in 2004). The summary of the epidemiological studies is quoted below:
Epidemiological studies have not identified an increased risk of cardiovascular disease associated with chlorinated or chloraminated drinking-water. Studies of other disinfectants have not been conducted.
The epidemiological evidence is insufficient to support a causal relationship between bladder cancer and long-term exposure to chlorinated drinking-water,THMs, chloroform or other THM species. The epidemiological evidence is inconclusive and equivocal for an association between colon cancer and long-term exposure to chlorinated drinking-water, THMs, chloroform or other THM species. The information is insufficient to allow an evaluation of the observed risks for rectal cancer and risks for other cancers observed in single analytical studies.
Studies have considered exposures to chlorinated drinking-water, THMs or THM species and various adverse outcomes of pregnancy. A scientific panel recently convened by the US Environmental Protection Agency reviewed the epidemiological studies and concluded that the results of currently published studies do not provide convincing evidence that chlorinated water or THMs cause adverse pregnancy outcomes.
Now if one looks at the details in the long document, one will find individual epidemiology studies showing links to a variety of cancers and that is what many pseudo-science websites selling products such as water filters or alternative disinfectants all show. However, if you look at the details, you find that nearly every study that shows a correlation has another study that shows no correlation and for many that do show something, the odds ratios are low and all are less than 2 meaning doubled risk. When this sort of mixture of studies happens and where odds ratios are fairly low, it often means that there are confounding variables people aren't taking into account. For example, if you find a correlation of lower cancer risk with people drinking well water compared to chlorinated municipal water, it may be due to well water being in rural areas where more people work on farms or doing heavier labor for more exercise compared to people in cities/suburbs. Unless the study explicitly looks for exercise or physical labor as a variable, they will falsely attribute cancer risk to chlorinated water rather than the lower level of exercise.
Your Links
The
first link you gave has a lot of incorrect information in it. The link talks about chloroform, but that's a non-issue as described in numerous papers I listed in
this post I linked to earlier (below toxic levels, it doesn't cause cancer and is removed by the body). They also talk about asthma and I already linked to
the thread that has recent data showing less and less of a link of swimming to asthma essentially overturning Bernard's original papers.
The
second link refers to a newer paper and I just bought it to read it in full. One quote from this paper is that "this study demonstrated that brominated acetic acids have more adverse health effects than chlorinated ones" which is consistent with what I wrote earlier. Some papers from
Barcelona, Spain were with water that was higher in bromide content though this latest paper has lower bromide content (< 0.1 ppm indoor; < 0.2 ppm outdoor). The paper showed that for HAA5, the major route of exposure is ingestion (i.e. swallowing pool water) and only about 5% was from inhalation and 1% from dermal absorption. The pool water averaged the following:
COMPOUND ..... INDOOR ..... OUTDOOR ..... WHO GUIDELINE ..... EPA LIFETIME CANCER RISK (using "OUTDOOR" values)
TCCA .............. 105 µg/L ..... 122 µg/L .......... 200 µg/L ...............
1 in 4100
DCAA ................ 71 µg/L ..... 154 µg/L ............ 50 µg/L ...............
1 in 4500
MCAA ................ 20 µg/L ....... 25.5 µg/L ......... 20 µg/L ............... (not available)
Understand that these WHO guidelines and EPA cancer risks are for drinking 2 quarts or liters of water every day for a lifetime. However, there is no way one is ingesting 2 quarts of pool water every day when swimming and as the study noted 94% of the exposure came from ingestion so the cancer risk is likely to be lower by a factor of 10 if not 100 (i.e. into the 1 in 100,000 range). Remember also that these are high bather-load commercial/public pools that generally have higher levels of disinfection by-products compared to outdoor residential low bather-load pools. The indoor pools likely did not have CYA so their active chlorine levels were 10 times higher than in residential pools (and it's possible the outdoor pools weren't using CYA since the reported FC levels were rather low and they were using a chlorinating liquid feed system).
The
third link says things like "trihalomethanes which are highly toxic, about 10,000 times more toxic than chlorine", but that is really saying more about how chlorine itself isn't very harmful and he doesn't say how the quantity of trihalomethanes in concentration is orders of magnitude smaller than that of the chlorine that produced it so the factor of 10,000 is really misleading in multiple ways. It's just a scare tactic. Yes, THMs can cause cancer, but one needs to look at the actual rates just as I did for the HAA5 above. I've had THM measurements made of my pool water and tap water and though the pool water concentrations are higher, they are roughly in the range of less than 1 in 50,000 cancer risk and that's if I drank 2 quarts of pool water every day for a lifetime. Now the website talks about showers and it is true that with hot showers one absorbs more disinfection by-products in the tap water through inhalation of aerosols and volatization of gasses and the pores of the skin are opened more so that there is more dermal absorption as well. However, even with these increased rates of absorption you are still looking at increased cancer risks that are still much lower than the 1 in 10,000 range.
So then one may look at some bladder cancer studies showing up to 2.0 odds ratios for those with the highest THM exposure and wonder what's going on, but that's when one needs to look at multiple studies as in the WHO report and also examine specific recent studies such as
this one and look at Table 1 that shows odds ratios of 3.05 for former smokers and 6.41 for current smokers that stands out rather significantly and being far more important that showering, swimming or bathing. Table 2 that shows the odds ratios vs. THM exposure is strange because it doesn't show an increase based on the number of years of exposure, only based on the average residential THM level during exposure and the same is true in Table 3 for the lifetime exposure amount for swimming. Also note in Table 3 that showering with moderate THM levels for women has a low 0.4 odds ratio
implying that women should shower with moderate levels of THMs to cut their bladder cancer risk by 60%! When one sees this sort of thing, one immediately thinks "confounding variables". In other words, there may be something else accounting for the variation such as people with access to swimming pools may have other kinds of correlated activity or exposure that accounts for the difference. This is why the WHO report generally discounted the bladder studies because they varied so much and were far from conclusive (but they do list them and their details so you can read about them).
