Trihalomethanes (THMs) are by-products that are formed when chlorine that is used in disinfecting drinking water interacts with organic matter that occurs naturally in the water. Adding chlorine to water leads to production of several byproducts that are classified as either halogenated or non-halogenated byproducts. These byproducts are THMs and haloacetic acids. THMs are a group of chemicals that are typically known as disinfection by-products and may also result from reactions between bromine and organic matter found in water that is being treated.
Epidemiological studies have linked THMs with serious health effects and most governments have taken steps towards limiting the quantity allowed in drinking water. Even so, THMs are just one group among numerous other possible disinfection by-products with majority of these not being monitored. In the US, the Environmental Protection Agency limits the concentration of four major THM constituents namely chlorofoam, bromodicloromethane, bromoform and dibromochloromethane to 80 parts per billion in treated water. The four THM constituents are known as t
otal trihalomethanes (TTHM). Most trihalomethanes are used in industries as refrigerants or solvents.
Formation of THMs
THMs are formed when chlorine interacts with organic matter such as river weeds, algae, and decaying leaves that are found in water. As chemical compounds, trihalomethanes form when three out of the four hydrogen atoms of methane are replaced by halogen atoms. Trihalomethanes that have similar halogen atoms are known as haloforms. Reaction between residual chlorine molecules and harmless organic matter result to formation of a group of these
chlorinated chemical compounds. Though THMs are odorless and tasteless, they are harmful and potentially toxic. The amount of byproducts formed is dependent on factors like the kind and quantity of organic matter present in the water, pH levels, temperature, contact time for chlorine, amount of chlorine added to the water and concentration levels of bromide in the water. Organic matter that results to THMs formation consists mainly of:
Levels of THMs in Water
- Humic Substances: This is the organic portion of soil that is left after extended microbial decomposition that is formed as a result of decaying wood, leaves or other vegetation matter
- Fulvic Acid: These are water soluble substances that have low molecular weight that are derived from humus.
Concentration of byproducts is largely determined by the levels of organic matter in water sources. Water facilities whose sources are surface water such as rivers, lakes and reservoirs tend to generate higher levels of THMs compared to facilities that use groundwater sources like springs and wells. Total Trihalomethanes (TTHM) concentrations generally range between 0.001 and 0.010 milligrams per liter in ground water and 0.030 and 0.0150 milligrams per liter in surface water. The distribution of the four THM compounds (chlorofoam, bromodicloromethane, bromoform and dibromochloromethane) varies broadly depending on bromide concentration levels in water.
Exposure to THMs
Chloroform, a constituent of THM is created in swimming pools that are disinfected using chlorine. Chloroform can also be as a result of haloform reaction where hypochlorites interact with organic substances such as sweat, urine, skin particles and hair. Some THMs are highly volatile and vaporize easily into the air. This makes it easy for people to inhale THMs when taking a shower or during other forms of interactions with contaminated water. The Environmental Protection Agency (EPA) has noted that this form of exposure has minimal effects compared to exposure that results from consumption. THM uptake is higher among swimmers particularly through dermal
absorption that occurs through the skin. This form of absorption represents 80 percent of THM exposure. Swimming in chlorinated pools tends to increase toxicity where chlorine byproduct’s toxic effects are much higher among younger swimmers compared to older swimmers. Studies conducted among adolescents reflect that an inverse relationship exists between the duration of time spent in chlorinated pools and serum testosterone levels.
A study conducted by academic and government researchers has added to previous evidence that shows that inhalation and dermal absorption of THMs during daily tap water usage can lead to higher THM concentration levels in the blood than just taking water that contains THM. The outcome of this study serves as a benchmark for epidemiologic investigations to be conducted in future that focus on the link between THMs present in tap water and serious health effects among human beings.
Trihalomethanes and Human Health
According to the Environmental Protection Agency, trihalomethanes (THMs) are present in many chlorinated water supplies. Though they pose a lower acute risk to people’s health compared to waterborne diseases, THMs still rank among the most important water quality challenges in the U.S. Trihalomethanes have been linked to negative health effects such as serious reproductive outcomes and cancer. A report generated by the University of Florida shows that exposure to THMs increases risk of cancer. Rebekah Grossman noted that two constituents of THMs namely dibromochloromethane and chloroform are carcinogens. Bromodichloromethane, which is also a constituent of THM is a mutagen that alters DNA.
Mutagens are known to affect the genetic composition of future generations and they are also carcinogenic. According to a study conducted in California, THMs may also be responsible for miscarriage and other reproductive problems. This study shows that the rate of miscarriage among women who ingested 5 or more glasses of cold water whose TTHM concentration was 0.075 milligrams per liter was 15.7% compared to 9.5% among women who ingested water with lower TTHM levels. Further, TTHMs have been linked to increased risks to bladder cancer and damage to major body organs including kidney, heart, lung, liver and the central nervous system.
Regulation of THMs
Previous THM regulations applied only to large water systems that server more than 10,000 people. The Environmental Protection Agency held that exempting smaller water systems had little or no negative effect on the health of smaller communities because most small water systems have groundwater as their source and also smaller systems use lesser detention time compared to larger systems which means less contact time between chlorine and organic matter. The EPA also thought that smaller communities lacked the necessary professional expertise to regulate THMs and the need for disinfection and such systems often use lower quantities of chlorine.
Currently, the EPA regulates TTHM for smaller communities as part of Microbial or Disinfection Byproducts (M/DBP) Rules. TTHM concentration levels allowed under these rules are 0.080 milligrams per liter of water and plans are underway to reduce these limits further to 0.040 milligrams.
Ways of Reducing THMs
There are several ways through which drinking water systems can reduce THM levels in water. Such include:
- Reducing the amount of organic matter in the water prior to chlorinating the water. Water treatment techniques like sedimentation, coagulation and filtration can help in removing most organic matter from the water. Where quantities of organic matter in water are huge, activated carbon technique can be used to rid the water of organic materials.
- Optimizing the usage of chlorine
- Changing the point at which chlorine is added to the water during the water treatment process. For instance, if chlorine is added after filtration or sedimentation, then the level of THM production is significantly reduced because these processes help rid the water of parts of organic material.
- Using alternative methods of disinfecting water such as chloramine, which is a mixture of chlorine and ammonia. This reduces the formation of THM because chloramine disinfects and does not result in formation of THMs. Also, Ozone may be used in addition to chloramine and chlorine. Chlorine dioxide is also another alternative to disinfecting water. This combination of disinfectants lowers THMs formation and also maintains residual concentration in the water distribution system. However, changing the type of disinfectants can change the entire water treatment process and may have an effect on the removal of other contaminants present in the water.
- Other methods of treating water include aeration, filtration, distillation, boiling, commercial home treatment or filter systems, nano-filtration, leaving tap water in a pitcher overnight in a fridge and activated carbon filtering.