Exposure science is the study of the effects of environmental factors on living organisms. Exposures include chemicals, metals, and radiation in the air, water, and soil, as well as biological agents, such as microbial toxins. Exposure science examines pathways and levels of exposure and their effects on human and ecosystem health.
Burgeoning human population and industrial development have led to an ever-increasing release of chemicals and other substances into the environment. The purpose of exposure science is to prevent, minimize, and manage the risks to humans and ecosystems from these environmental factors. Exposure science identifies sources of exposure and pathways that chemicals follow from sources into ecosystem and the human body. Exposure science measures levels of contaminants in the environment and in humans and other organisms and analyzes their effects. Exposure science is particularly concerned with the links between environmental exposure and human disease, and the effects of genetic and lifestyle factors on those links.
Many chemicals that have long been thought to be harmless are turning up in unexpected places, including the human body, with unknown or unexpected effects. Exposure science is used to predict how new chemicals will behave in and travel through the environment and to identify potential risks. It can inform the design of safer chemicals and products and improved guidelines for safer use. Public knowledge of exposure science can help individuals, communities, and societies make informed decisions about their use of chemicals.
Studies of occupational diseases date back to at least 1700. By the 1920s, scientists were measuring workplace air pollution and connecting it to the development of human disease. By the mid-twentieth century, scientists were investigating exposures to and assessing risks from pollutants in outdoor and indoor air and water. During the 1970s, with the establishment of the U.S. Occupational Safety and Health Administration (OSHA) and the U.S. Environmental Protection Agency (EPA), exposure science diverged into two paths: one focusing on workplace exposure and the other focusing on exposure of the general public to environmental pollutants.
By the 1980s, molecular epidemiologists were beginning to link interactions between environmental and genetic influences with biological indicators—biomarkers—in individuals. Personal exposure studies originally measured external chemicals that could be inhaled, ingested, or absorbed into the body through the skin. By the twenty-first century, exposure science included measuring levels of chemicals in biological tissues and fluids and the use of models to predict exposure levels under a variety of circumstances.
Exposure science took on new significance with the completion of the Human Genome Project, because genome-association studies failed to explain the variability in human diseases, indicating that environmental exposures played a major role. However, unlike the human genome, it is impossible to characterize an individual's total environmental exposure. In 2005, Christopher Wild coined the term exposome to describe an individual's lifetime exposure to environmental factors, including lifestyle factors, beginning in the womb. The exposome includes both external exposures, such as pollution, food, and radiation, and internal exposures from infection, inflammation, and the microbiome—the diverse collection of billions of microorganisms that reside in the human body.
Breathing air, drinking water, eating an apple, touching a surface, walking across a carpet, or using a common household product can expose humans to a wide variety of natural and manmade chemicals that can enter and travel through the body. There are usually multiple sources of exposure to a specific substance. Thus, assessing exposure is extremely complex, and the development of new assessment and measurement methods is a fundamental aspect of exposure science. With improved analytical chemistry methods, extremely low levels of many chemicals can be detected in the human body and in the environment.
Exposure science develops sophisticated computer methods and models for estimating total exposures and risks associated with chemicals encountered in daily life. The EPA's Stochastic Human Exposure and Dose Simulation (SHEDS) model is used to estimate ranges of total chemical exposure in a population via different pathways (inhalation, ingestion, skin contact), based on data from dietary and human activity databases and levels of chemicals measured in food, water, air, soil, and on surfaces. Exposure estimates from SHEDS are used in further models, such as the physiologically based pharmacokinetic (PBPK) model that predicts how chemicals will travel through the body and concentrate (bioaccumulate) in tissues and fluids. SHEDS has been used to develop regulatory guidelines for organophosphate and carbamate pesticides and for chromated copper arsenate, a wood preservative used in playground equipment. The EPA was using SHEDS-PBPK models to assess potential human and ecosystem risks from pyrethroid pesticides.
Exposure levels and pathways provide little information about impacts on human health, which usually depend on multiple factors—genetic susceptibility, lifestyle, diet, and various physical and medical conditions—that determine an individual's biological response. Environmental exposures are known to contribute to many common human diseases, including obesity, asthma, cancer, and neurodegenerative disease. However, evidence from exposure science indicates that individual biological responses are often more important than the exposure itself. As of 2012, such responses were very difficult to measure or assess. The influence of exposure on the development and/or progression of disease depends not just on individual biological responses, but also on the type and degree of exposure at specific ages and developmental stages.
Exposures that may be harmless in adults can have serious consequences in children.
Exposure science is the foundation of public health protections. Unfortunately, voluntary phaseouts of hazardous chemicals, restrictions on their use, or outright bans have often led to increased use of alternative chemicals, which exposure science eventually reveals to have their own toxic effects. Furthermore, chemicals that are banned for specific uses may be reintroduced for new purposes. Therefore, exposure science increasingly focuses on predicting potential routes of exposure and bioaccumulation before chemicals are introduced.
Only a tiny fraction of the more than 10,000 chemicals in current commercial use and the hundreds of new ones introduced each year have been analyzed by exposure science. For example, the use of flame retardants called polybrominated diphenyl ethers (PBDEs) was restricted after they were accidentally added to animal feed, resulting in livestock deaths and high levels of human exposure. The flame retardants tris-BP and chlorinated tris were banned from children's clothing after they were found to be bioaccumulating through skin contact. However, all of these compounds are still used in textiles and other products. By the late 1990s, high concentrations of PBDEs were found in breast milk, with household dust a major route of human exposure. Chlorinated tris and other flame retardants also bioaccumulate in the human body through ingestion and inhalation of dust. Perfluorooctane sulfonate (PFOS), used in various products including food packaging, has bioaccumulated in the human body, in wildlife, and in environments worldwide. Perfluorinated compounds that have replaced PFOS are being measured in household dust. Bisphenol A (BPA), used in food and drink packaging, has bioaccumulated in the bodies of more than 90% of the U.S. population.
The chemical complexity of crude oil and fuel products and the widespread effects of oil spills on marine ecosystems pose major challenges for exposure science. It is difficult to predict the impacts of events such as the 2011 Deepwater Horizon blowout in the Gulf of Mexico. It is even harder to assess the impacts of small oil spills that occur daily around the world. Toxic chemicals in crude oil also constantly enter waterways through vehicle exhaust and storm water runoff from streets and parking lots. The thousands of chemicals that may be present in a particular oil make it difficult to assess their effects in the tissues of marine organisms. The most extensive exposure science study of the effects of crude oil had been the aftermath of the 1989 Exxon Valdez oil spill in Alaska's Prince William Sound.
With the dissemination of anthrax spores through the mail in 2001, exposure science focused on biological toxins. Anthrax is a global disease caused by the bacterium Bacillus anthracis. Because anthrax is a bioterrorism threat, the science focused on routes of entry into the human body, including inhalation, which has a mortality rate of more than 90%. Exposure science developed rapid and sensitive automated technologies for measuring anthrax toxin at very low levels, as early as 12 hours after exposure—before symptoms develop and while the disease is treatable. These developments were expected to be applicable to other infectious diseases.
Asbestos was among the first major successes for exposure science. Asbestos includes six natural mineral fibers that have been used in thousands of products. These fibers separate into smaller fibers that can be inhaled, causing disease and death from asbestosis, lung cancer, and mesothelioma. As of the early 2000s, mining of asbestos had been halted in the United States and imports had fallen to 3% of their 1973 peak, although people continued to die from prior exposure, and occupational exposure still occurred during maintenance and remediation of older buildings. Exposure science also revealed the dangers of secondhand asbestos exposure and began examining exposures to asbestos-like materials called elongated mineral particles (EMPs).
Measurements of airborne nicotine and particulate matter (PM) from tobacco smoke revealed the extent of exposure to SHS and has led to smoke-free legislation around the world. SHS exposure science has demonstrated that there is no safe level of exposure and that nonsmoking areas and ventilation systems are insufficient for protecting the health of nonsmokers.
Exposure science has long been concerned with air pollution. New methods for measuring polluting gases, total suspended particles, particle sizes, and sulfates, and the application of advanced statistical methods to new and historical studies, have demonstrated that there is no threshold below which PM concentrations can be considered safe.
Other major areas of exposure science research include the following:
See also Air pollution ; Anthrax ; Asbestos ; Asbestosis ; Asthma ; Bioterrorism ; Centers for Disease Control and Prevention ; Environmental Protection Agency (EPA) ; Exxon Valdez ; National Institute for Occupational Safety and Health ; Occupational Safety and Health Administration ; Radiation ; Radiation exposure .
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International Society of Exposure Science Secretariat, c/o JSI Research and Training Institute, 44 Farnsworth St., Boston, MA, 02201, (617) 482-9485, Fax: (617) 482-0617, http://isesweb.org .
National Institute of Environmental Health Sciences, PO Box 12233, MD K3-16, Research Triangle Park, NC, 27709, (919) 541-3345, Fax: (919) 541-4395, email@example.com, http://www.niehs.nih.gov .
U.S. Environmental Protection Agency, 4601M, Ariel Rios Bldg., 1200 Pennsylvania Ave. NW, Washington, DC, 20460, (202) 564-3750, firstname.lastname@example.org, http://www.epa.gov .
Margaret Alic, PhD