For decades, toxicology studies and regulatory approaches have focused on the effects of single chemicals on human health. However, scientists recognize that we are exposed to complex mixtures of environmental chemicals every day and throughout our lifetimes. We encounter these mixtures through the air we breathe, the food and water we consume, and the products we use in our home or on our bodies.
These complex chemical mixtures may have greater effects on our health than each chemical would alone. For example, exposure to ozone and aldehydes, which are components of smog, produces greater health effects than would be predicted based on each chemical alone.
NIEHS and the National Toxicology Program (NTP) are developing methods to study environmental exposures in ways that more closely represent the mixture of exposures that we experience in real life.
Studying a chemical mixture requires evaluating the effects of individual chemicals within the mixture, interactions among those chemicals, and the combined effect the mixture may have on human health. Researchers are examining how these mixtures interact in cells, animals, and humans to determine health effects. In addition, they are developing innovative statistical methods to assess the health effects of mixtures and characterize real-life exposures.
What Is NIEHS Doing?
Identifying Health Effects
At any given time, we are exposed to complex, real-world mixtures of potentially dangerous compounds. These mixtures include chemicals and infectious agents, which can act in combination to create more severe effects. For example, carriers of the hepatitis B virus who are also exposed to aflatoxin show a 60-fold increase in the incidence of liver cancer.
In recent years, scientists at NTP as well as those affiliated with NIEHS intramural and extramural programs have made substantial progress in learning how exposures to various mixtures affect human health. The following are a few of their findings.
- Autoimmunity. A review of 109 chemicals and mixtures found some compounds that seemed to be protective, and a few that appeared to be risk factors, for the development of autoimmune disease. The biggest potential risk was exposure to an antibacterial and antifungal agent called triclosan, which was banned by the U.S. Food and Drug Administration in 2017.
- Diabetes. Exposure to the metals arsenic, manganese, and cadmium can impair the function of pancreatic beta cells – a change that is linked to type 2 diabetes risk.
- Neurodevelopment. Exposure to a mixture of suspected endocrine-disrupting chemicals early in pregnancy may reduce IQ in children. These chemicals can interfere with hormone action, even at low levels.
- Obesity. A study of both individual and joint effects of different pesticides indicated that exposure before birth can generate different effects on body weight and composition in boys and girls. Maternal DDT exposure was associated with higher weight in girls whereas maternal pyrethroid exposure was linked to lower weight in boys.
- Preterm birth. Analysis of trace amounts of metals, both individually and in mixtures, in urine samples of pregnant women showed links to a higher risk of preterm birth. The study suggested a cumulative effect of urinary arsenic, mercury, and tin.
- Sexual development. Prenatal exposure to chemicals used in unconventional oil and gas extraction, also known as fracking, altered mammary gland development in female mice. The researchers saw effects from a mixture of 23 commonly used extraction chemicals at levels that were observed in drinking water in regions with fracking operations.
- Women’s health. High levels of synthetic chemicals known as per- and polyfluoroalkyl substances (PFAS) in the body were tied to earlier menopause in women. Accelerating menopause by even a few years can have significant effects on cardiovascular and bone health, quality of life, and overall health among women.
NIEHS Research Efforts
Though real-world chemical exposures occur in mixtures, researchers have historically studied exposures to single chemicals. Studies of mixtures are tricky because of the need for complicated measurements, the difficulty of determining which components are of concern, and the need for clearer understanding and application of available statistical methods.
In human health studies, real-world environmental exposures occur in mixtures that are too complex for traditional statistical models. Not only are there many factors to consider simultaneously, but they are often related to each other, may interact, and may involve complex biological processes.
In laboratory studies such as those led by NTP, scientists conduct research on chemical mixtures using the following three study designs, based on materials and outcomes of interest:
- The component-based approach focuses on individual chemicals in a mixture, such as polycyclic aromatic hydrocarbons from oil spills. Health effects are generally assessed by adding up the individual chemical concentrations.
- The whole mixture approach concentrates on complex mixtures with no single chemical of interest, such as botanical dietary supplements. This approach uses a concept called sufficient similarity, which assumes that if two chemicals are similar enough, the toxicity data from one can be applied to the other.
- The systems biology approach evaluates mixtures based on how exposures affect human health outcomes such as cardiovascular disease and cancer. Results can help prioritize chemicals for risk assessment studies.
New toxicology tools developed by NIEHS-funded researchers and collaborators can help assess how exposures to chemical mixtures affect human health. The following are a few examples:
- A new data-driven approach analyzes various types of toxicity data to prioritize chemicals for further testing. Scientists have used the method to investigate the toxicity of polycyclic aromatic compounds, which arise from burning or decomposition of organic matter.
- A “sufficient similarity” method allows scientists to apply findings from a tested substance to related substances with a similar but not identical composition. For example, NTP researchers evaluated the similarity of numerous variations of botanical supplements containing black cohosh (Actaea racemose), Echinacea purpurea, and Ginkgo biloba with well-established NTP test samples.
- A chemical profiling technique combines experimental and computational information to categorize complex substances, including mixtures. The method combines chemical and biological data by using a series of cell models to screen chemicals for similarities in biological activity, which indicates whether a substance will affect living cells or tissues. The new approach could help minimize toxicity testing in animals, especially when assessing chemical alternatives.
- A sophisticated tool using high-resolution mass spectrometry helps scientists overcome research challenges posed by complex, real-world exposures. The technique can test samples from tissue, plasma, urine, dust, and water for thousands of chemicals at once.
- A new computational method could provide better evidence for public health interventions that target multiple chemical exposures at the same time. Dietary recommendations, changes in consumer behavior, and regulations of industrial sources are examples of such interventions.
A strategic approach to mixtures research has been developed over many years. The cross-divisional NIEHS Combined Exposure and Mixtures Working Group, formed in 2010, gathers scientists from across the institute with an interest in mixtures research. This working group meets regularly and serves as a forum for discussing and fostering collaboration on mixtures projects.
CEM research was included as a strategic goal in the 2012–2017 NIEHS Strategic Plan and in the current NIEHS Strategic Plan (2018– 2023). One goal of CEM is to prioritize mixtures research based on cancer, cardiovascular disease, and other diseases relevant to humans. That approach differs from others that tend to only consider similarities among environmental agents in their chemical structure or mechanism of action.
Projects undertaken by the CEM program will help provide the scientific basis for public health decision-making by informing:
- Selection of chemicals for inclusion in cumulative risk assessments using knowledge of biological pathways.
- Consideration of component-based versus whole-mixture approaches to minimize uncertainty in hazard evaluation.
- Application of methods for predicting mixture toxicity using data from constituents, determining sufficient similarity of whole mixtures, evaluating pharmacokinetics of multicomponent mixtures, and identifying active constituents.
Through the Powering Research through Innovative Methods for mixtures in Epidemiology (PRIME) program, NIEHS funds researchers who are developing methods that incorporate toxicological information into statistical models. PRIME grantees study how exposure to mixtures of metals, pesticides, endocrine disrupting chemicals, persistent organic pollutants, and air pollution affect health. Some researchers also examine how non-chemical exposures, like stress and nutrition, may amplify or protect against the adverse health effects of a chemical mixture.
PRIME projects are developing novel statistical approaches that:
- Enable research to discover the biological pathways that link an exposure to a disease.
- Determine which exposures within a mixture are most harmful, and how the mixture as a whole can influence health.
- Incorporate information on chemical toxicity to provide biological context for exposures.
- Integrate the timing of exposures, such as during critical windows of development when individuals are more susceptible to exposures.
- Share data that captures the variability in exposures and outcomes across space, time, and populations.
The NIEHS Superfund Research Program also supports research of mixtures on human health and the environment. For example, the Texas A&M University Superfund Research Program (SRP) Center studies exposure to chemical mixtures during environmental emergencies and has been involved in the response to Hurricanes Harvey and Florence.
The Outstanding New Environmental Health Science (ONES) awards are geared towards researchers at formative stages of their careers, many of whom are studying health effects resulting from exposure to environmental mixtures. For example, ONES awardee Andres Cardenas, Ph.D., from the University of California, Berkeley, studies the extent to which exposures to neurotoxic mixtures just before and after birth, as well as key prenatal nutrients, can jointly influence brain development. Another ONES awardee, Jessie Buckley, Ph.D., from Johns Hopkins University, is investigating the role of environmental chemical mixtures in the development of childhood obesity.
Stories from the Environmental Factor (NIEHS newsletter)
- Novel Toxicity Assessment Strategies Highlighted at DNTP Board Meeting (July 2021)
- Innovative Research Into Cancer, Mixtures Headlines DNTP Board Meeting (March 2021)
- Cutting-edge Approaches to Mixtures Research Shared at NIEHS Event (November 2020)
- Mixtures Researchers Have New Tool for Health Effects Studies (May 2020)
- Chemical mixtures in source water and drinking water – A resource created by the World Health Organization (WHO) to help guide assessing and managing risk to human health associated with exposure to chemical mixtures from drinking-water and its sources.
- Guidelines for the health risk assessment of chemical mixtures – A procedural guide created by the Environmental Protection Agency (EPA) for evaluating data on the health risks from exposures to chemical mixtures.
- Linking Chemical and Nonchemical Mixtures to Health Disparities (February 2021) – NIEHS grant recipient Rachel Morello-Frosh, Ph.D., conducts research on how social factors may interact with environmental chemical exposures to influence health disparities in certain populations.
Cross Divisional Seminar: Improving Mixtures Health Assessments: Combining In Silico with In Vitro / In Vivo Models to Evaluate Co-occurring Contaminants
- Cross Divisional Seminar: Improving Mixtures Health Assessments: In Vitro Assays for (Bio)monitoring of Chemical Mixtures – From the Environment to People
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