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Toxic Tuesdays

How the Duration of Exposure Affects Toxicity

Toxic Tuesdays

CHEJ highlights several toxic chemicals and the communities fighting to keep their citizens safe from harm.

How the Duration of Exposure Affects Toxicity

CHEJ has previously written about the importance of considering multiple chemical exposures when assessing the toxicity of exposure to toxic chemicals. In addition, it is also important to consider the duration of exposure. How long was a person exposed? What was the concentration of the substance(s) during this period of time are critical to truly understanding the cumulative effects that a person has suffered? Without this information, we can only partially understand the risks of exposure to toxic chemicals. 

Yet when evaluating whether exposures to toxic chemicals pose risks to human health, the government’s approach is usually very narrow: it assumes there is a single chemical from a single source at a single point in time with a single exposure pathway causing a single health effect. This approach makes risk assessment more feasible and understandable, but it does not reflect the reality of our lives.

In reality, we are exposed to multiple chemicals at a time and exposures can happen over a long period of time. This means that considering the potential effects of a single exposure to a single chemical isn’t sufficient for evaluating public health risks. We need to include cumulative risks that account for both multiple chemical exposures and exposure over time in order to begin to understand the risks to public health. But incorporating these parameters into a risk assessment poses significant new challenges that  requires more estimates and generates additional uncertainty than the traditional risk assessment approach.

In many cases, exposure assessments assume that exposure to a chemical happens in a single instant in time. In some cases – like cancer risk – EPA assumes that risk is proportional to the lifetime dose. In general, longer exposure means greater risk, but the relationship between duration of exposure and health risk is complicated. The risk depends on the effects an exposure has on the body and the body’s response to it. In some cases, the body may adapt to exposure and the threat over time may be less than additive. In other cases, the body may become more sensitive and the threat over time may be more than additive. Repeated exposure can also influence health risk: past exposure to some chemicals can make us more vulnerable to subsequent exposure. And how do you consider the effects caused by exposure to multiple chemicals that target the same organ in the body can cause more damage than exposure to any of those chemicals individually?

The effect of exposure over time is important to consider in risk assessments, but agencies like EPA and ATSDR do not have comprehensive frameworks for how to assess this cumulative risk. Part of the reason for this is a lack of data. The guideline values we use to evaluate risks are driven by data generated from exposures to a single chemical for a defined period of time. For common chemicals and chemical mixtures that people are exposed to, we need to know how different concentrations and durations of exposure affect health. There is a need for more scientific study on how exposures to chemicals over long periods of time can impact our risk for adverse health effects. Once people have been exposed to chemicals, we also need better tools to measure their past exposure so it can be accounted for in risk assessments.

While more research and data is crucial, there will always be uncertainty in science and data, and we cannot let uncertainty stop us from taking action to protect health. In addition to more scientific study, we need clearer guidelines from federal agencies regarding how to consider cumulative risk – both from multiple chemical exposures and exposures over time – in evaluating if an environmental hazard is a threat to human health. We also need to acknowledge how poor the tools we have are at considering cumulative risks caused by exposures to multiple chemicals over time.

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1,2-dichloroethane (1,2-D)

Toxic Tuesdays

CHEJ highlights several toxic chemicals and the communities fighting to keep their citizens safe from harm.

1,2-dichloroethane (1,2-D)

1,2-dichloroethane (1,2-D) – also called ethylene dichloride – is a clear, oily liquid with a sweet smell that is man-made and not found in nature. It is used in the production of plastic and vinyl products like polyvinyl chloride (PVC) pipes, upholstery, automobile parts, and housewares. It is also added to the leaded gasoline used in airplanes and racecars. 1,2-D was previously used in some household products like carpet cleaners, but most of these products are discontinued. 1,2-D can enter the environment during its production, disposal, or use. It can enter the water and soil, but because it is volatile (meaning it readily evaporates), most 1,2-D ends up in the air. Once in the air, it can persist for many months and travel long distances.

Because most 1,2-D in the environment ends up in the air, people are most likely to be exposed to it by breathing contaminated air. Exposure can cause damage to many organ systems: brain dysfunction such as nausea and blurred vision; gastrointestinal dysfunction such as vomiting, gastritis, and colitis; respiratory dysfunction such as difficulty breathing and bronchitis; immune system dysfunction such as decreased ability to fight infection and decreased blood clotting; liver damage; and kidney damage. In extreme cases, exposure can cause heart attack and death. In studies of laboratory animals, exposure also caused lung, liver, brain, and reproductive cancers. Based on this research, the US Environmental Protection Agency has determined that 1,2-D probably causes cancer in humans.

1,2-D is known to be a very dangerous chemical with multiple harmful effects on human health. It is a positive development that household products with 1,2-D are largely discontinued. However, its continued use in products like PVC pipes and leaded gasoline mean that 1,2-D and its threats to human health remain pervasive in both household and industrial environments.

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Lead in Public Housing

Toxic Tuesdays

CHEJ highlights several toxic chemicals and the communities fighting to keep their citizens safe from harm.

Lead in Public Housing

Lead is a naturally occurring metal that has been used in many household products like paint and plumbing materials. This makes people most likely to be exposed to lead in their own homes, through ingesting or inhaling contaminated paint, dust, or water. Lead exposure affects all organs but is particularly damaging to the brain, causing defects in learning and memory. Children are especially vulnerable to lead exposure because of their growing brains, and exposure can cause defects in brain development, behavioral problems, and irreversible learning disabilities. Even though it’s been known for over two hundred years that lead is toxic, it is estimated that 800 million children worldwide are exposed to lead today. (CHEJ has previously written about the health effects of lead exposure here.)

A new study has found that access to federal housing assistance is associated with lower blood lead levels (BLLs), demonstrating how housing access influences health. The US Department of Housing and Urban Development (HUD) has three main housing assistance programs that help 5 million low-income households access affordable, high-quality housing:

  1. The public housing program provides subsidized housing units at a specific site that is owned by the local public housing authority.
  2. The multifamily income-restricted housing program provides subsidized housing units at a specific site that is owned by a private entity.
  3. Tenant-based housing choice vouchers (HCVs) provides subsidies for tenants to use towards finding housing in the private market.

The study authors linked HUD administrative records to data from an existing survey that measured people’s health including their BLLs. This allowed the authors to connect people’s BLLs to whether or not they were enrolled in a HUD housing program. To determine if access to HUD housing programs was associated with lower BLLs, the authors compared those who were enrolled in a HUD program to those who were not enrolled but would become enrolled within the next 2 years. This ensured that the groups being compared were similar in their socio-economic status and eligibility for HUD housing assistance. Overall, the study sample included over four thousand people.

The authors found that when controlling for demographic factors like race, ethnicity, sex, age, partnership status, and households size, average BLL was 11.4% lower for people enrolled in HUD housing programs compared to people who were not enrolled at the time. The effect was biggest for people enrolled in public housing programs. The effect was smallest for people enrolled in the HCV program. The authors hypothesize that this protective effect of HUD housing assistance is because HUD has stricter compliance and enforcement of federal lead-paint laws – such as the Lead-Paint Poisoning Prevention Act, the Residential Lead-Based Paint Hazard Reduction Act, and the Lead-Safe Housing Rule – in their public-owned housing units compared to housing units that are privately owned. Because the HCV program has recipients find housing on the private market, this may explain why there was little effect on BLLs for people enrolled in HCV. The authors also note that as housing construction has slowed in the past few decades, affordable housing options on the private market tend to be older construction that are more likely to contain lead-based paint and pipes. HUD’s required inspections, maintenance, abatement, and clearance activities seem to be effective at decreasing people’s exposure to lead. This is consistent with previous studies that have found other positive health outcomes associated with public housing.

The authors found that the association between HUD housing program enrollment and lower BLLs was strongest for non-Hispanic white people. The association was much lower for Black and Mexican American people. While the study cannot explain why this is, the authors offer several explanations rooted in historical and ongoing racism:

“Black households continue to face significant barriers to high-quality housing and high-opportunity neighborhoods that may have fewer lead hazards because of legacies of racist housing policies and urban planning practices in the United States. These practices include redlining, zoning and land use restrictions, gerrymandering of school and census boundaries, predatory lending, and urban renewal initiatives in Black and Brown neighborhoods that displaced families and built highways, airports, and other large pollution-emitting sources in their neighborhoods through eminent domain.”

Overall, this study indicates that housing through HUD programs protects against lead exposure. This is likely a success story of regulations that require inspection, abatement, and removal of lead in public housing; it suggests that requiring units on the private housing market to adhere to these same regulations could have a significant impact on lead exposure in the United States. Because lead is one of the worst toxic chemicals with the potential to do lifelong damage to children, public policy efforts that reduce lead exposure should be a priority. The fact that the lead protective effect of HUD programs is less substantial for nonwhite people demonstrates how systemic racism impacts housing and health. This study shows that housing justice and environmental justice are deeply intertwined: access to high-quality housing is crucial for health and safety. The study also shows that neither housing justice nor environmental justice can be achieved without racial justice.

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Dealing with Uncertainty When Evaluating Toxicity​

Toxic Tuesdays

CHEJ highlights several toxic chemicals and the communities fighting to keep their citizens safe from harm.

Dealing with Uncertainty When Evaluating Toxicity

In a recent issue, we discussed the many challenges in evaluating the adverse health effects that result from exposure to a mixture of toxic chemicals. Despite this, scientists still estimate and assess risks by attempting to compensate for these uncertainties.

This is done by assigning an uncertainty factor (UF) to the different uncertainties. How well these uncertainty factors fill in the gap in what we do not know is a matter of controversy and opinion. Especially when you acknowledge that we only have good toxicity information on about 1% of the more than 80,000 chemicals that are in use.

Consider just a few of the uncertainties. The first step in assessing risks is to determine what substances a person was exposed to, at what concentration and for how long. Rarely is this information ever available, so assumptions need to be made to estimate this critical information. Sometimes, there is limited air, soil or water data. This data is often collected for a different purpose, such as to evaluate the need for remediation as opposed to evaluating public health risks. There are also uncertainties in how the samples were collected, the accuracies of and precision of the analytical measurements and the thoroughness of the sampling (were the samples taken at the right places, analyzed for the right substances and at relevant concentrations). At times, modeling is used to estimate how much of a chemical a person was exposed to (usually after making assumptions about even what kind of chemicals a person was exposed to), how long they were exposed and at what concentration.

The next step is to evaluate the toxicity information available on the chemical in question. This would include information from animal studies, clinical trials and epidemiological studies involving people. Since most of the data that exists is from animal studies, this step already creates enormous uncertainties. These uncertainties include extrapolating results in animals to people; the variability in response among people; the sensitivity in response among people; estimating acute or short-term responses in people when the only data you have is from chronic or long-term exposure, and vice versa. These examples just touch the surface of the many uncertainties in our understanding of how chemicals affect a person’s health. 

Another factor that comes into play is the health status of the individual who was exposed. People who are generally healthy and without pre-existing conditions respond differently to toxic chemicals than people with prior exposures, poor immune or nutritional status, or pre-existing health problems.

To address these many uncertainties, scientists have developed what were originally called safety factors, but now are referred to simply as uncertainty factors (UF). These uncertainty factors can range from 1 to 10 and often are multiplied together to yield a composite uncertainty factor that can be as high as 100 (10 x 10). These UFs are included in the estimate of the risks a person or group of people face.

Scientists give an UF to each specific uncertainty trying to compensate for the uncertainty. Doing this requires making many assumptions about areas of knowledge that very little is known about. These assumptions are made by “scientific experts” who very quickly become convinced that they “know” the health risks that a person or a group of people face. Of course, they do not really know. Instead, what they have is an opinion based on multiple assumptions, typically for a single substance.

What compounds this process is that the people who make these risk assessment estimates are scientific experts, and do not include the people who have to bear the risks of the chemical exposures. That’s not right! The people who bear the risks need to be involved in the risk assessment and health evaluation process because of the many uncertainties that exist in estimating exposures and in extrapolating what little data exist to evaluate adverse health effects resulting from exposures to low level mixtures of toxic chemicals.

For more about uncertainties when evaluating the adverse effects from chemical exposures, see Environmental Decisions in the Face of Uncertainty, by the Institute of Medicine of the National Academies, 2013.

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1,4-dichlorobenzene (1,4-DCB)

Toxic Tuesdays

CHEJ highlights several toxic chemicals and the communities fighting to keep their citizens safe from harm.

1,4-dichlorobenzene (1,4-DCB)

1,4-dichlorobenzene (1,4-DCB) – also known as p-dichlorobenzene (p-DCB) – is a colorless solid chemical that readily evaporates into the air. 1,4-DCB does not occur in nature, and it is often produced for use in deodorants or disinfectants because it has a strong odor that humans can smell at very low concentrations. It is commonly used in household products like mothballs and deodorizing sprays. It also has industrial uses as a pesticide ingredient and a precursor to commercial dyes. 1,4-DCB can enter the environment through its household uses, pesticides, and industrial waste disposal. 1,4-DCB mostly enters the environment as a vapor, and people are likely to inhale it in homes and buildings where it is used. Solid 1,4-DCB can also bind to soil and remain there for long periods of time, but people are less likely to be exposed to it in this way.

Inhaling high concentrations of 1,4-DCB can cause irritation or burning sensations in the eyes and nose. It can also cause coughing, nausea, difficulty breathing, dizziness, headaches, and liver dysfunction. Touching products that contain 1,4-DCB can also cause burning sensations on the skin. In studies of laboratory animals, 1,4-DCB exposure caused liver, kidney, and blood defects as well as liver cancer. The US Department of Health and Human Services and the International Agency for Research on Cancer both classify it as being reasonably anticipated to cause cancer.

Because of the danger to human health, the Environmental Protection Agency has set a maximum 1,4-DCB concentration that can be present in drinking water without observing adverse health effects. The European Union has gone even further, banning use of 1,4-DCB in mothballs and air fresheners because of its potential to cause cancer. Similar regulation in the US could protect more people from the health risks of 1,4-DCB exposure.

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Barium

Toxic Tuesdays

CHEJ highlights several toxic chemicals and the communities fighting to keep their citizens safe from harm.

Barium

Barium is a silver-colored metal which is found in the earth in compounds with other elements. Many barium compounds have industrial uses: barium sulfate is used as a drilling lubricant by the oil and gas industries to facilitate drilling through rock; barium carbonate is a rat poison; and barium oxide is used in the production of electronics and glass. Barium can enter the air through the production of barium-containing compounds and the improper disposal of barium-containing waste. It can then enter the soil and water. Barium compounds that do not dissolve in water can persist in the soil and water for a long time.

People are most likely to be exposed to barium by drinking contaminated water. Barium compounds that do not dissolve in water – like barium sulfate and barium carbonate – are not known to be harmful to human health. However, barium compounds that do dissolve in water – like barium chloride and barium hydroxide – can harm human health because they release barium ions into the body. Barium ions interfere with the normal electrical impulses generated in the brain, muscles, and heart. Exposure can cause gastrointestinal dysfunction such as vomiting, diarrhea, and abdominal cramps. It can also cause anxiety, disorientation, difficulty breathing, decreased blood pressure, numbness, muscle weakness, and paralysis. The eyes, immune system, respiratory system, and skin can also be damaged by barium exposure. The Environmental Protection Agency (EPA) has set a limit on how much barium can safely be in drinking water, but almost 800 Superfund sites are known to have barium contamination, suggesting that there may be potential for barium exposure in some communities.

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What Scientists Know and Don’t Know About Exposures to Low Level Mixtures of Toxic Chemicals

Toxic Tuesdays

CHEJ highlights several toxic chemicals and the communities fighting to keep their citizens safe from harm.

What Scientists Know and Don’t Know About Exposures to Low Level Mixtures of Toxic Chemicals

Not long ago, the Huffington Post ran a story called: A Roll of the Dice: The Unknown Threat of Exposures to Chemical Mixtures, by Chris D’Angelo that talked about the difficulties scientists are having in answering the questions about adverse health effects following the horrific train derailment in East Palestine, OH more than a year ago. It’s an important article for anyone dealing with a toxic chemical exposure issue, especially in a community setting. 

It’s important because it gets to the heart of the science – what scientists know and don’t know about low level multiple chemical exposures to toxic chemicals such as occurred in East Palestine and many other contaminated sites around the country. In most cases, people are exposed to multiple chemicals simultaneously at low concentrations over various periods of time. Rarely are people exposed to just one chemical.

Yet when the government steps in to assess the health risks at these sites, they use the best tool available to them – risk assessments based on peer reviewed published data. The article discusses why this approach is very limited in what it can tell about the risks people face from exposure to multiple chemicals at low concentrations. Risk assessment is limited because virtually all of the published peer reviewed data addresses exposure to only a single chemical at a time and that very little data exists to inform what happens when people are exposed to multiple chemicals at low concentrations. Linda Birnbaum, former director of the National Institute of Environmental Health Sciences, told D’Angelo that mixtures are a complex problem that has long frustrated the field of toxicology.

The risk assessment process relies on this limited scientific data because it’s all we have to assess health risks. D’Angelo points this out arguing that data derived from exposure to one chemical at a time bears no relationship to the actual risks people face in the real world such as in East Palestine. He describes it this way: “In communities like East Palestine, Ohio, where residents were exposed to potentially dozens of different chemicals following the fiery derailment of a Norfolk Southern train in February, environmental agencies are often quick to declare the air, water, and soil safe, despite having little grasp of how substances could be interacting to harm human health.”

D’Angelo points out that the “…dangers in East Palestine may not be any one chemical but several working in tandem. And the fields of toxicology and epidemiology remain largely incapable of investigating and understanding that threat.”

But instead of acknowledging what the science actually tells us about exposures to low level mixtures of toxic chemicals, government, in the case of East Palestine, has released disingenuous and misleading statements meant to reassure the public that everything is alright and taking no action to address the adverse health symptoms that the people in East Palestine are continuing to experience including nose bleeds, headaches, skin rashes and breathing difficulties.

If the EPA and other health agencies were honest and truthful with the public, they would tell the people of East Palestine that they really don’t know the true exposure risks, that scientists don’t know very much about what happens to people exposed to low level mixtures of toxic chemicals. While perhaps not reassuring, the truth allows everyone to better understand what’ they are facing.  

The article concludes with a way forward by suggesting that EPA should follow the lead of what the government did to take care of Vietnam Veterans who were exposed to Agent Orange and the soldiers exposed to emissions from the burn pits in Iraq and Afghanistan, among others. In these cases, soldiers do not have to prove that their illnesses were caused by their exposure to toxic chemicals. If they can show that they were exposed, that’s sufficient for them to get health care and other compensation.

Communities like East Palestine shouldn’t be held to a different standard, especially given the many unknowns about the toxic exposures caused by the train derailment. In the absence of a basic understanding of what adverse health effects might result from exposures to the mixtures of toxic chemicals released into the community by the train derailment, the government should take steps to move the people of East Palestine who want to move, provide health care for those who were exposed and establish a medical monitoring program to follow these people.

These steps will begin the long and difficult process of acknowledging what we know and don’t know about exposes to low level mixtures of toxic chemicals and begin to learn what happens to the people exposed in these situations. Read the full article here.

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Carbon Monoxide

Toxic Tuesdays

CHEJ highlights several toxic chemicals and the communities fighting to keep their citizens safe from harm.

Carbon Monoxide

Carbon monoxide is a toxic gas that is difficult to detect because it has no smell, taste, or color. It can be produced from both natural and human-made sources when carbon fuel – such as gasoline, wood, coal, charcoal, propane, natural gas, or trash – is incompletely burned. The most common source of carbon monoxide in outdoor air is exhaust from gas-powered vehicles. It can also be produced in indoor air through house fires or use of gas-powered appliances such as portable generators, furnaces, water heaters, stoves, and fireplaces. Carbon monoxide is also produced in industrial chemical manufacturing to create a group of plastics called polycarbonates.

When carbon monoxide enters the air it can remain there for several months. Inhaling air contaminated with carbon monoxide interferes with red blood cells’ ability to carry oxygen throughout the body. This can cause difficulty with breathing, headache, nausea, dizziness, vomiting, vision impairment, confusion, and chest pain. In high doses it can cause seizures, coma, and death. Exposure to high doses while pregnant can also cause miscarriage. People with heart or lung diseases are particularly vulnerable to the effects of carbon monoxide exposure. Even once exposure to carbon monoxide has ended, there can be long-term effects on heart and brain function.

Because of the extreme toxicity of carbon monoxide, the Environmental Protection Agency (EPA) sets standards for safe levels of carbon monoxide in the air. Despite these standards, studies estimate that 50,000 people in the United States need emergency medical treatment for carbon monoxide exposure each year, and that about 1,000 die from carbon monoxide exposure each year. Carbon monoxide has also been found in many Superfund sites identified by the EPA. These realities indicate that more stringent standards, testing, and regulations may be necessary to keep people safe from carbon monoxide.

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Acrolein

Toxic Tuesdays

CHEJ highlights several toxic chemicals and the communities fighting to keep their citizens safe from harm.

Acrolein

Acrolein is a toxic chemical that presents itself as a clear to yellowish liquid that evaporates quickly and is highly flammable. As it vaporizes, it has an unpleasant smell and tends to accumulate in low areas since it is heavier than air. Acrolein is used as a precursor ingredient in many different kinds of manufacturing industries including plastics, paint, leather finishings, and paper coatings. It is also used as a biocide to control plant and algae growth in water systems.

Acrolein exposure usually occurs in the form of inhalation. Acrolein is formed from the combustion of certain organic compounds. As such, it is commonly formed from the burning of fossil fuels, animal and vegetable fats, and tobacco. It is a common, albeit minimal, by-product of forest fires.

The health effects of short-term exposures to acrolein are fairly well understood. Acrolein is severely irritating to skin, eyes, and mucous membranes. If inhaled, it causes respiratory distress, an asthma-like reaction, and delayed pulmonary swelling. Contact with the skin or eyes produces irritation and lacrimation, and can result in chemical burns.

The long-term health effects of acrolein are much less studied. There are some indications that prolonged exposure can cause respiratory sensitization, a process through which exposure to a chemical leads to hypersensitivity of the airways when exposed again to the same or similar chemicals. Potential adverse reproductive effects or links to cancer have not been explored well enough to draw any conclusions.

It is perhaps this uncertainty over long-term health effects that most concerns residents of East Palestine, OH. After the train derailment dumped more than 1 million pounds of various industrial chemicals in the community, authorities responded by removing some of the contamination and performing controlled burns on the rest. These activities have released dangerous levels of acrolein into the air, as an analysis of EPA data by Texas A&M researchers revealed. Despite accurately assessing the immediate health impacts of acrolein on the community, it is a shame that the same researchers then downplayed the risks of prolonged exposure by saying that “it would take months, if not years, of exposure to the pollutants for serious health effects.” This is simply not true, as we have very little information about long-term exposure to even low levels of acrolein. The situation in East Palestine is extremely worrisome, and researchers downplaying the health risks the community is facing is very counterproductive to the situation.

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Trihalomethanes (THMs)

Toxic Tuesdays

CHEJ highlights several toxic chemicals and the communities fighting to keep their citizens safe from harm.

Trihalomethanes (THMs)

Trihalomethanes (THMs) are a class of chemical compounds which contain three halogen atoms. Common THMs include chloroform, fluoroform, and chlorodifluoromethane. While THMs are used in some industrial processes like refrigeration, people are most likely to be exposed to them through drinking water. Most water utilities use small amounts of chlorine as a disinfectant to keep water supplies clean. While adding chlorine is generally accepted to be an important practice to protect public health, this chlorine can react with organic matter in the water and create THMs. Drinking contaminated water is generally considered to be the most serious route of exposure to THMs, though one study has found that bathing with contaminated water causes even higher exposure. THMs are odorless and flavorless, so people may not know if they have been exposed.

Many types of THMs have adverse health effects upon exposure, and there are four that are known to be particularly harmful to human health:

  1. Chloroform is known to cause lung, liver, and kidney damage, and in high doses can lead to death. In studies of laboratory animals, drinking chloroform-contaminated water caused liver and kidney cancer. The Environmental Protection Agency (EPA) classifies it as a likely cancer-causing agent in humans. 
  2. Bromoform can cause neurological impairments, unconsciousness, and death. In studies of laboratory animals, drinking bromoform-contaminated water caused liver and kidney cancer. EPA classifies it as a probable cancer-causing agent in humans.
  3. Dibromochloromethane is less well understood, but in studies of laboratory animals, drinking contaminated water caused liver and kidney cancer. EPA classifies it as a possible cancer-causing agent in humans.
  4. Bromodichloromethane is known to cause liver, kidney, and immune system damage. It can also cause reproductive system damage and lead to miscarriage and low birth weight. In studies of laboratory animals, drinking bromodichloromethane-contaminated water caused intestinal, kidney, and liver cancer. EPA classifies it as a probable cancer-causing agent in humans.

Because of the danger of THM exposure, EPA regulates the maximum amount of total THMs allowed in tap water. While this can help keep people safe, regulating the total THMs may not necessarily be effective because each type of THM can cause health effects on their own. Furthermore, little is known about the health effects of simultaneous exposure to multiple THMs. EPA has maximum contaminant level goals (MCLGs) for each of the four THMs listed above, but these goals are not enforced by laws or regulations, so they have no power to keep people safe. Because most people get their water from chlorinated municipal water supplies, and because THMs have such serious health effects, more must be done to keep people safe from exposure. This includes more studies to understand the effects of simultaneous exposure to multiple THMs and enforceable standards for individual THMs in drinking water.

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