In practice it has been shown that maintaining free residual chlorine concentrations between 0.2 and 1 mg / L in the water of the networks and reservoirs of the distribution systems protects the consumption water from biological recontamination; It is evident that the use of chlorine has been maintained over time by the important benefits it provides in the control and minimization of waterborne diseases, however it has the disadvantage of the formation of byproducts of disinfection (DBPs), compounds considered a public health problem due to its cytotoxic, genotoxic and cancer-causing character in laboratory tests on animals. The objective of the quality management in the supplies nowadays is to guarantee the disinfection with chlorine or other disinfectants, minimizing the contents of DBPs.

Numerous studies have found a relationship between the consumption of chlorinated drinking water and certain negative effects on human health such as cancer of the bladder, colon, stomach, pancreas, kidney and rectum, as well as adverse effects for pregnancy and the newborn.

Risk assessments have shown that the mutagenic, carcinogenic and teratogenic effects can not be attributed exclusively to the regulated DBPs but also to the emerging DBPs, which in many cases have much higher toxicities than the regulated DBPs despite their lower concentration. Nitrogen disinfection byproducts (N-DBPs), iodinated DBPs (I-DBPs) and brominated disinfection byproducts (Br-DBPs) are significantly toxic. However, despite the numerous studies conducted in the last forty years, many researchers disagree on the effects of exposure to DBPs in drinking water and there is still no clear evidence on the effects of drinking water DBPs on diseases of long latency like cancer. However, clearer health effects have been described in swimming pool waters, where concentrations of DBPs are higher, such as respiratory problems in pool workers and asthma among professional swimmers. There is mixed evidence linking pool attendance and respiratory health among children, although an increasing number of studies suggest that the benefits of swimming in swimming pools outweigh the risks associated with exposure to DBPs

I-DBPs include iodoacids and iodo-trihalomethanes, which may be present in drinking water at very low concentrations from ng / L to μg / L. Iodine trihalomethanes have been described as cytotoxic potentials. Recent studies have attributed to these compounds, greater cytotoxic and genotoxic effects than their brominated and chlorinated analogues, due to the greater potential remanence of the iodine atom. Among the species of I-THMs the iodoform is the most cytotoxic and the chlorodiiodomethane the most genotoxic. Additionally proven evidence of cell toxicity of I-DBPs, a cytotoxicity level much higher than that of bromoform and iodoform chloroform.

Thanks for the comments. The risk, if any, issue is not clear at all. 2 of the THMs are not carcinogens in drinking water (EPA) and 3 are not according to WHO and the 4th (BDCM) will likely be determined to not be carcinogenic based upon published high dose animal tests. The THMs were carcinogenic in corn oil gavage dosing but not when dosed in drinking water. Probably a combination of dose rate and corn oil effects in the rodents.It appears that the animal test design are flawed relative to drinking water exposures.A few DBPs found at ppb or less are possibly carcinogens. The other large group is HAAs which are much more interesting than THMs. The trace iodos are cytotoxic, which means that they are chemically reactive to naked cells. However, their reactivity means that they probably disappear in the GI tract. Iodoform is not carcinogenic in whole animal tests.

Bladder cancer is the only cancer that had some correlation with THMs is some epi studies (about 10% of the 75+ publications).

You might be interested in this paper.

National Trends of Bladder Cancer and Trihalomethanes in Drinking Water:A Review and Multicountry Ecological Study. Joseph A. Cotruvo and Heather Amato.

Dose Response Journal January 23 17(1) Open Access.

ABSTRACT We examined trends in incidence of bladder cancer in 8 countries in the 45 years since trihalomethanes (THMs) were detected in chlorinated drinking water. Total trihalomethanes (TTHMs) are the principal regulated disinfection by-products (DBPs) along with halogenated acetic acids (HAAs). Numerous epidemiological studies have examined exposure to TTHMs and associations with bladder cancer. Concentrations of TTHM have declined in most of the 8 countries that were studied as has smoking prevalence. Incidences of bladder cancer have usually stayed relatively flat, especially for females, with some variations. SinceTHMs are not carcinogens in whole animal tests, they may not be appropriate surrogates for studying potential cancer risks in drinking water. Etiology of bladder cancer is complex; incidence correlates with age. Previously identified risk factors include smoking, type 2 diabetes, sex, ethnicity, arsenic, aromatic amines, and occupations. As a predominant risk factor, smoking trends may dominate incidence rates, but additional time might be required to determine whether a DBP risk exists due to long latency periods. Causal drinking water-related bladder cancer risks remain questionable and likely small compared to other factors,although surrogate-based DBP management is an appropriate strategy for maintaining drinking water quality as long as it does not compromise microbial disinfection.

I remember seeing a conference of Steve Hrudey several years ago where he asked the audience (with an electronic survey) if THM caused cancer. The vast majority answered affirmatively. I think that most water professionals are unaware that it is more chlorinated water which has been linked to cancer rather than THM.

Benoit, I agree that most water people still believe that THMs in drinking water are carcinogenic, even though the bioassay data do not support that. The chlorinated water connection is also weak, although there are some DBPs that test carcinogenic at high doses. Most of the epi studies have attempted to correlate cancers, esp bladder, with THMs. My conclusion in the Dose Response 8 country article that covered at least 40 years, is that the exposure variable is impossible to quantify accurately, so that the relatively few reported positive correlations by others could have been generated by the estimates that are chosen by the study authors. The bladder cancer correlations with THM trends were non existent in most countries. Bladder cancer incidence was a flat line in the US and Canada while THMs have been declining. Sweden's is increasing while THMs are historically low, and chlorination practices have not changed. Australia's are decreasing while THMs are much higher than most other countries. If the THMs are not carcinogenic, do they correlate with other more potent DBP chemicals that are concurrently present at much lower concentrations? Probably not.

To date, numerous works have been published that find evidence of negative effects for the health of THMs and DBPs dissolved in drinking water. But there have also been published numerous works that do not find a correlation between diseases such as cancer and the THMs contained in drinking water. It is true that in the last decade the concentrations of THMs and other DBPs in drinking water have decreased considerably in Europe and other countries, which could suggest the need for a review of epidemiological studies. But taking into account the discrepancies of opinion on the effects of THMs on health, to avoid health risks derived from these compounds it seems evident that it is convenient to apply appropriate water treatment processes for their minimization.

I would like to add that the control of THMs in the supply systems can be used as a quality indicator that helps to achieve the overall improvement of the quality of drinking water. It is not possible to reduce the concentration of THMs in the water of a distribution system without decreasing other quality parameters such as TOC, turbidity, Fe, Mn, Al, ammonium.

Dominguez, Yes!. We do not need to invent hypothetical minute cancer risks as a reason to provide good quality drinking water. It keeps a lot of researchers busy looking for the last ppt of something or doing more epi studies on top of the 75 or more inconclusive published studies on the books. And it scares the public for no reason. It would be more productive to emphasize providing good technology to produce good drinking water. However, the major drinking water issue is distribution systems and plumbing regrowth(legonellosis etc) . You can produce wonderful drinking water at the treatment plant and then put it into a decrepit distribution system. It does not reach the tap the same as when it left the plant. Joe

Yes. we agree on the need to have conclusive studies and accepted by the scientific community to clarify whether the THMs and other DBPs dissolved in drinking water are carcinogenic or not. And it would also be necessary to adapt the technical-sanitary regulation of drinking water to these conclusions. To date, this seems a utopia and consequently a sanitary regulation of waters with restrictive limits of concentrations of THMs in drinking water is maintained, without knowing for sure whether they affect health or not. But, what is the best solution? In this situation, drinking water managers have no alternative but to continue treating the water with advanced processes to keep the concentrations of DBPs at low levels, as required by the regulations.

Obviously, we agree that it is not necessary to invent any hypothetical risk of cancer, although we will also have to be prudent until we have clear conclusions. In my opinion it is more important to maintain the disinfection in the supply networks than the possible effects for the health of the associated DBPs.

We also agree that many of the problems of drinking water quality are due to distribution networks.

Charles D. Goss you might find the discussion interesting

Thanks for the comments. It is true that disinfectants have provided safety against pathogens in the past and that the risk of THM is inconclusive at concentrations we are exposed to from disinfection. However it should be noted that THM is just an indicator of several known and unknown Br-, Cl-, N-DBP etc with unknown toxicity. It is always difficult to prove through epi studies given multiple causative factors with drinking water contributing only a minute fraction of what we are generally exposed to through food. This should not in anyway be used to revise guidelines because the guidelines allow us to minimise the exposure and to create awareness amongst water operators/professionals to be cautious and minimise the exposure as low as they could. May be in the light of inconclusive studies regulatory pressure on water utilities could be softened.

I also agree on the fact that microbial growth/contamination within the distribution system has the highest risk and application of disinfectants have reduced the risk of diseases substantially in the past century or so. Recent microbial water quality in disinfected water systems have shown abundant potentially opportunistic pathogens ( Psuedomonas aeroginosa, Legienella, Mycobacterium etc) in high chlorine/chloramine residuals. If they are thriving why not an opportunistic pathogen with real risk survive in water supply system? This leads to the thinking that if disinfectants are actually providing the safety or it is the integrity of water supply system that provides the safety nowadays. We need to take note here that there are many customers who are chronically drinking water with low disinfectant residuals and some supply systems operate without secondary disinfectants.

Overall, it is hard to prove DBP are toxic at levels majority (there are some who exposed to incredibly high DBP levels ) of the customers are exposed to. Similarly it is hard to prove the disinfectant levels or the integrity are the ones that provides safety against pathogen regrowth/intrusion. Please note that I am talking about the secondary disinfection not the primary disinfection which is essential.

Thanks for your thoughtful comments. We agree that the bottom line is that disinfection is essential, and it should never be compromised, as WHO has repeatedly said. There has been too much negative talk about disinfectants and DBP risks, among both opportunistic researchers, and uninformed press and some regulators who do not understand. That has resulted in sickness and death. For example, about 25 years ago there were thousands of illnesses and deaths in South America because some thoughtless people stopped chlorinating because they decided that chlorination caused cancer. I wrote the original THM regulation in 1979. It was carefully designed to protect disinfection, while reducing unnecessary DBPs. I still support it, but we must understand that the reality is that after all of these years and dozens of studies, DBPs in drinking water have not been demonstrated to cause cancers. My published article this year demonstrated that there was no relationship between THMs and bladder cancer incidence in 8 countries over those 40 plus years. . The risk is negligible, if at all.

Thanks for the comments.

Reading your post a couple things come to mind....THMs are not the only by-products to think about in regards to using hypochlorite to disinfectant drinking water. If the concentrated hypochlorite solutions used to dose are not protected from degradation (heat and UV) perchlorate can be formed. Also regarding corn oil gavage - is this product tested for pesticides and herbicides? like glyphosate?

Peter,

There are numerous trace organic DBPs formed usually at low ppb or ppt. Chlorate and perchlorate are present in hypochlorite and their concentrations are a function of the age and storage conditions. So, it is best to use fairly fresh and diluted hypochlorite and store it in a cool place out of the sun. The perchlorate issue has been overblown. It does compete with iodide in the NIS transport to the thyroid, but so do many other common anions. Perchlorate binding is about the strongest. The key is to have appropriate iodide/iodate in the diet. Even EPA's Inspector General issued a report that stated that, rather than regulating perchlorate in drinking water, it would be more beneficial and cost effective to assure that prenatal care always include iodine supplementation. Of course, adequate dietary iodine is also essential for the general public for many reasons.

Congratulations Prof. you are doing a very great job. All the best!