Water is essential and naturally contains minerals that nourish our bodies. However, due to human impact, many other substances—such as organic compounds, industrial chemicals, and added disinfectants—are also found in tap water. This post takes a closer look at some of the primary chemicals and by-products commonly present in tap water and their potential effects on health.

Why and how much Chlorine is Added to Tap Water

In many countries, including Japan, chlorine is used to disinfect water, eliminating harmful bacteria and viruses. Chlorine disinfection is essential for public health but has its trade-offs.

the World Health Organization (WHO) allows up to 5 mg/L of chlorine in drinking water, while Japan’s standards requires the chlorine residual level to be range from 0.1 mg/L to 1 mg/L. In Tokyo, actual chlorine levels in tap water often measure average 0.4 mg/L, while U.S. standards permit up to 4 mg/L, with average levels in cities like San Francisco around 2.5 mg/L.

By-products of Chlorine Disinfection: Trihalomethanes (THMs)

In the 1970s, researchers discovered that when chlorine mixes with organic matter in water, it forms by-products known as trihalomethanes (THMs), including chloroform. Studies link THMs to various health risks, including cancer, liver and kidney damage, immune dysfunction, and even congenital abnormalities. Common types of THMs include:

– Chloroform (CHCl3)**

– Bromodichloromethane (BDCM, CHCl2Br)**

– Dibromochloromethane (DBCM, CHClBr2)**

– Bromoform (CHBr3)**

Regulations vary widely; in the U.S., the EPA caps THMs at 0.08 mg/L, while in Japan, the limit is 0.1 mg/L. Some environmental organizations, like the Environmental Working Group (EWG), advocate for stricter limits, recommending 0.015 mg/L.

There is growing concern about long-term exposure to THMs. Studies suggest that frequent swimming in chlorinated pools may increase the risk of asthma and allergies in children, and adults exposed to chlorine in pools could have a higher risk of bladder, rectal cancers, and cardiovascular issues.

“Cancer risk among people using chlorinated water is as much as 93 percent higher than among those whose water does not contain chlorine,” according to the U.S. Council of Environmental Quality.

According to the BreastCancerFund.org,“One common factor among women with breast cancer is that they all have 50 to 60 percent higher levels of these chlorination by-products (THMs) in their fat tissue than women without breast cancer . . .”

Arsenic in Groundwater

Arsenic is a naturally occurring toxin in soil and rocks, but it also dissolves into groundwater. Even small amounts are highly toxic, and it’s classified as a carcinogen.

To limit exposure, both Japan and the U.S. enforce a maximum of 0.01 mg/L for arsenic in drinking water. Tokyo’s Shibuya district, for instance, tests much lower, around 0.001 mg/L.

Fluoride: Beneficial or Harmful?

Fluoride, commonly found in natural water sources, is added to water supplies in some countries to prevent cavities. However, recent studies from the U.K. and Canada challenge its benefits, suggesting that fluoride may actually disrupt thyroid function, contribute to reproductive issues, and affect children’s development.

WHO recommends up to 1.5 mg/L, the U.S. EPA permits up to 4.0 mg/L. Japan’s limit for fluoride in tap water is 0.8 mg/L, and levels in Tokyo’s Shibuya district are typically low, around 0.1 mg/L.

British researchers report in the British Medical Journal that fluoridation studies are flawed. A Canadian Government report found fluoridation does more harm than good. A US National Institutes of Health Panel found most tooth decay studies, including hundreds on fluoride, scientifically invalid. Even UNICEF, the organization that protects children, reports, “more and more scientists are now seriously questioning the benefits of fluoride, even in small amounts.”

NRC has warned that doses of fluoride (0.01-0.03 mg/kg/day) achievable by drinking fluoridated water, may reduce the function of the thyroid among individuals with low-iodine intake. Reduction of thyroid activity can lead to loss of mental acuity, depression and weight gain.

In 1990 fluoride was found to be an equivocal carcinogen by the National Cancer Institute Toxicology Program.

Risk to the thyroid gland. According to the NRC, fluoride is an “endocrine disrupter.” Most notably, the NRC has warned that doses of fluoride (0.01-0.03 mg/kg/day) achievable by drinking fluoridated water, may reduce the function of the thyroid among individuals with low-iodine intake. Reduction of thyroid activity can lead to loss of mental acuity, depression and weight gain.

Pesticides and Other Contaminants

Pesticides, like glyphosate from herbicides, often end up in water supplies and may carry health risks. Glyphosate, a key ingredient in the herbicide Roundup, has been found in drinking water in various locations and has been linked to cancer in some studies.

Conclusion

I just picked up some of the contaminants in water here. There are many others including heavy metals, and radiations, microplastic, even antibiotics has been found in some areas. While tap water generally meets safety standards, the contaminants are very local dependent. So we all should be alert about what is in our water. Its long-term effects on health are a growing concern.

In the next episode, I will write about how we can mitigate the risks associating our tap water.