8.3: Environmental Toxicology

Introduction

Just because something is a chemical does not make it bad. After all, water is a chemical, air is a chemical, you are made of chemicals. The same goes for something that is labeled “natural” (which, by the way, has no legal definition). Arsenic, cyanide, and radiation are natural. None of those are good for you.

In this section, we will focus on environmental hazards. Some of these are natural, some contain natural ingredients, and some are produced entirely by humans. The toxicological hazards described below are those to which humans can be exposed and are often used to change aspects of our environment.

Linking the Environment and Human Health

Environmental health profoundly influences human health through an intricate relationship and various pathways that shape the well-being of individuals and communities.

Industrialized society is irresponsible in its treatment of natural systems. The consequences of such actions eventually impact human populations’ health and well-being. 

The health of our environment is a crucial aspect of our overall well-being. As awareness of environmental pollution grows, it is essential to develop methods for measuring pollutants in our environment.

Environmental Agents and Hazards

Environmental hazards and exposure are a growing concern, with factors such as temperature and pollution affecting the health of individuals and populations. Accurate information and risk management models are crucial for understanding and addressing environmental risks and empowering individuals and communities to take action.

Environmental agents are specific substances that can directly interact with, and potentially impact, the health of the environment and humans. Environmental agents can be categorized into biological, chemical, and physical. The sources of environmental agents are diverse, ranging from human activities to natural disasters and biological events.

• Physical agents include noise, temperature, vibrations, radiation, and lighting, originating from various sources, including the built environment (human-made structures and surroundings) and industry.
• Chemical agents, including heavy metals, pesticides, polychlorinated biphenyls (PCBs), chlorofluorocarbons (CFCs), endocrine-disrupting chemicals (EDCs), and sulfur dioxide (SO2), are used in agriculture, industry, and in the burning of fossil fuels for transportation and electricity generation.
• Biological agents were the most significant environmental health hazard through much of human history and include molds, dust mites, cockroaches, pollen, bacteria, viruses, protozoa, parasitic worms, and pet dander, saliva, and waste.

Environmental hazards originate from environmental agents, including industrial emissions and human activities, and can adversely affect human and animal health.

Human activities are the primary drivers of environmental agents, and societal and lifestyle choices, including urbanization, economic activity, recreational pursuits, and food habits, play a role. The presence of these agents, or toxins, in the air, water, and land can have deleterious effects on human health across the lifespan.

Environmental agents can create environmental health hazards, or conditions that have the potential to cause harm to the environment and humans.

Environmental Hazards and Impacts on Health

Environmental agents can pose hazards by polluting the air we breathe, the water we drink, and the food we eat, affecting our quality of life and overall health. While some of these hazards, such as smoke emanating from stacks, are clearly visible, most are not.

For example, small plastic fragments such as microplastics (MPs) and nanoplastics (NPs) are ubiquitous pollutants in water, air, and soil. NPs are more hazardous due to their extremely small size (<1000 nm), which allows them to escape water treatment processes and float long distances in both air and water, contaminating fresh water, oceans, air, and food. Furthermore, their size enables them to penetrate living tissues and cells more easily.

These particles are generated for use in consumer products such as cosmetics, cleansers, and detergents (major sources) and through the degradation of plastics such as tires (Lai et al., 2022). Small plastic contaminants can transport pollutants, microorganisms, and pathogens on their surfaces, thereby amplifying their physiological and environmental effects. While understanding of the impact of small plastics on human health is only just beginning to emerge, toxicological studies indicate that NPs can cross the blood-brain, intestinal, and placental barriers.

What Makes Something Hazardous?

A hazardous substance is defined in federal government regulations as “one that may cause substantial personal injury or illness during reasonable handling or use, including possible ingestion by children.”

According to the Federal Hazardous Substances Act (FHSA), household products are hazardous if they contain substances that have one or more of the following hazardous properties:

• Corrosive: A product that can burn or destroy living tissue, such as skin or eyes, or by chemical action.
  • Examples: Drain cleaners, oven cleaners, and lye.
• Irritant: A product is an irritant if it is not corrosive and causes injury to the area of the body that it comes in contact with after immediate, prolonged, or repeated contact.
  • Examples: Toilet cleaners, chlorine bleach cleaners, and some pool chemicals.
• Strong Sensitizer: A product that can cause an allergic reaction upon repeated use of the same substance. Usually, this does not happen when a person first comes in contact with the product, but after a second exposure.
  • Examples: Dyes, oils, tars, rubber, soaps, cosmetics, perfume, insecticides, wood resins, plants, paints, plastics, glues, fiberglass, metals, and polishes.
• Flammable: Any substance, liquid, solid, or the contents of a self-pressurized container, like aerosol cans, that can be easily set on fire or ignited. Extremely flammable, flammable, and combustible are the three flammability categories based on testing.
  • Examples: Paint thinners, some solvents, adhesives, rubber cement, and hair spray.
• Toxic: A product is toxic if it can cause long-term effects like cancer, birth defects, or neurotoxicity (toxic to nerves).
  • Examples: Brake fluids, fungicides, insecticides, fertilizers, rat poison, and antifreeze.

Routes of Exposure to Chemicals

Any product is hazardous if it can cause personal injury or illness to humans. To cause health problems, chemicals must enter your body.

There are three main “routes of exposure,” or ways a hazard can get into your body.

• Ingestion: Eating or drinking hazardous substances or contaminated foods and water, and absorbing these substances through your gastrointestinal tract.
• Inhalation: Breathing in gases, vapors, and sprays that are absorbed through the lungs and enter the bloodstream.
• Dermal (skin or eye contact): Some hazardous products can be absorbed through the skin or your eyes and cause injuries.

Once chemicals have entered your body, some can move into your bloodstream and reach internal “target” organs, such as the lungs, liver, kidneys, or nervous system.

What Forms do Chemicals Take?

Chemical substances can take many forms. They can be in the form of solids, liquids, dusts, vapors, gases, fibers, mists, and fumes. The form a substance is in has a lot to do with how it gets into your body and what harm it can cause. A chemical can also change forms. For example, liquid solvents can evaporate and give off vapors that you can inhale. Sometimes chemicals are in a form that can’t be seen or smelled, so they can’t be easily detected.

What Health Effects Can Chemicals Cause?

An acute effect of a contaminant (The term “contaminant” means hazardous substances, pollutants, pollution, and chemicals) is one that occurs rapidly after exposure to a large amount of that substance. A chronic effect of a contaminant results from exposure to small amounts of a substance over a long period of time. In such a case, the effect may not be immediately obvious. Chronic effects are difficult to measure because they may not be apparent for years. Long-term exposure to cigarette smoking, low-level radiation exposure, and moderate alcohol use are all thought to produce chronic effects.

For centuries, scientists have known that just about any substance is toxic in sufficient quantities. For example, small amounts of selenium are required by living organisms for proper functioning, but large amounts may cause cancer. The effect of a certain chemical on an individual depends on the dose (amount) of the chemical. This relationship is often illustrated by a dose-response curve, which shows the relationship between dose and the response of the individual. Lethal doses in humans have been determined for many substances from information gathered from records of homicides, accidental poisonings, and testing on animals.

A dose that is lethal to 50% of a population of test animals is called the lethal dose-50% or LD-50. Determination of the LD-50 is required for new synthetic chemicals to provide a measure of their toxicity. A dose that causes 50% of a population to exhibit any significant response (e.g., hair loss, stunted development) is referred to as the effective dose-50% or ED-50. Some toxins have a threshold amount below which there is no apparent effect on the exposed population.

Environmental Contaminants

Contamination of air, water, or soil with potentially harmful substances can affect any person or community. Contaminants (Table 1) are often chemicals found in the environment in amounts higher than what would be there naturally.

Organisms can be exposed to these contaminants from a variety of residential, commercial, and industrial sources.

Sometimes, harmful environmental contaminants occur biologically, such as mold or a toxic algae bloom.

Table 1. Classification of Environmental Contaminants

Contaminant	Definition
Carcinogen	An agent that may produce cancer (uncontrolled cell growth), either by itself or in conjunction with another substance. Examples include formaldehyde, asbestos, radon, vinyl chloride, alcohol, and tobacco.
Teratogen	A substance that can cause physical defects in a developing embryo. Examples include alcohol and cigarette smoke.
Mutagen	A material that induces genetic changes (mutations) in the DNA. Examples include radioactive substances, x-rays and ultraviolet radiation.
Neurotoxicant	A substance that can cause an adverse effect on the chemistry, structure or function of the nervous system. Examples include lead and mercury.
Endocrine disruptor	A chemical that may interfere with the body’s endocrine (hormonal) system and produce adverse developmental, reproductive, neurological, and immune effects in both humans and wildlife. A wide range of substances, both natural and man-made, are thought to cause endocrine disruption, including pharmaceuticals, dioxin and dioxin-like compounds, arsenic, polychlorinated biphenyls (PCBs), DDT and other pesticides, PFAS, and plasticizers such as bisphenol A (BPA).

An Overview of Some Common Contaminants

Arsenic is a naturally occurring element that is widely distributed in our environment, including water, soil, dust, air, and food. Levels of arsenic can regionally vary due to farming and industrial activity, as well as natural geological processes. Arsenic from farming and smelting tends to bind strongly to soil and is expected to remain near the land's surface for hundreds of years, providing a long-term source of exposure. Wood that has been treated with chromated copper arsenate (CCA) is commonly found in decks and railings in existing homes and outdoor structures such as playground equipment. Some underground aquifers are located in rock or soil that has naturally high arsenic content.

Most arsenic enters the body through the ingestion of food or water. Arsenic in drinking water is a problem in many countries worldwide, including Bangladesh, Chile, China, Vietnam, Taiwan, India, and the United States. Arsenic may also be found in foods, including rice and some fish, due to uptake from soil and water. It can also enter the body by breathing dust containing arsenic. Researchers are finding that arsenic, even at low levels, can interfere with the body’s endocrine system. Arsenic is also a known human carcinogen associated with skin, lung, bladder, kidney, and liver cancer.

Mercury is a naturally occurring metal, a useful chemical in some products, and a potential health risk. Mercury exists in several forms; the types people are usually exposed to are methylmercury and elemental mercury. Elemental mercury at room temperature is a shiny, silver-white liquid that can produce a harmful odorless vapor. Methylmercury, an organic compound, can build up in the bodies of long-living, predatory fish. To keep mercury out of the fish we eat and the air we breathe, it’s important to take mercury-containing products to a hazardous waste facility for disposal. Common products sold today that contain small amounts of mercury include fluorescent lights and button-cell batteries.

Although fish and shellfish have many nutritional benefits, consuming large quantities of fish increases a person’s exposure to mercury. Pregnant women who eat fish high in mercury on a regular basis run the risk of permanently damaging their developing fetuses. Children born to these mothers may exhibit motor difficulties, sensory problems, and cognitive deficits. Figure \(\PageIndex{1}\) identifies the typical (average) amounts of mercury in commonly consumed commercial and sport-caught fish.

Figure \(\PageIndex{1}\). Mercury concentrations in fish can reach potentially dangerous levels (published by the Maine Center for Disease Control & Prevention).

Bisphenol A (BPA) is a chemical synthesized in large quantities for use primarily in the production of polycarbonate plastics and epoxy resins. Polycarbonate plastics have many applications, including use in some food and drink packaging, e.g., water and infant bottles, compact discs, impact-resistant safety equipment, and medical devices. Epoxy resins are used as lacquers to coat metal products such as food cans, bottle tops, and water supply pipes. Some dental sealants and composites may also contribute to BPA exposure. The primary source of exposure to BPA for most people is through the diet. Bisphenol A can leach into food from the protective internal epoxy resin coatings of canned foods and from consumer products such as polycarbonate tableware, food storage containers, water bottles, and baby bottles. The degree to which BPA leaches from polycarbonate bottles into liquid may depend more on the temperature of the liquid or bottle than on the age of the container. BPA can also be found in breast milk.

What can I do to prevent exposure to BPA?

Some animal studies suggest that infants and children may be the most vulnerable to BPA's effects. Parents and caregivers can make the personal choice to reduce exposures of their infants and children to BPA:

• Don’t microwave polycarbonate plastic food containers. Polycarbonate is strong and durable, but over time, it may break down from overuse at high temperatures.
• Plastic containers have recycling codes on the bottom. Some, but not all, plastics marked with recycle codes 3 or 7 may contain BPA.
• Reduce your use of canned foods.
• When possible, opt for glass, porcelain, or stainless steel containers, particularly for hot food or liquids.
• Use baby bottles that are BPA-free.

Phthalates are a group of synthetic chemicals used to soften and increase the flexibility of plastic and vinyl. Polyvinyl chloride becomes softer and more flexible when phthalates are added. Phthalates are used in hundreds of consumer products. Phthalates are used in cosmetics and personal care products, including perfumes, hair sprays, soaps, shampoos, nail polishes, and skin moisturizers. They are used in consumer products such as flexible plastic and vinyl toys, shower curtains, wallpaper, vinyl miniblinds, food packaging, and plastic wrap. Exposure to low levels of phthalates may come from eating food packaged in plastic containing phthalates or from breathing dust in rooms with vinyl miniblinds, wallpaper, or recently installed flooring that contains phthalates. We can be exposed to phthalates by drinking water contaminated with them. Phthalates are suspected to be endocrine disruptors.

Figure \(\PageIndex{2}\). Nail products are known to contain toxic chemicals, such as dibutyl phthalate (DBP), toluene, and formaldehyde.

Formaldehyde is a colorless, flammable gas or liquid that has a pungent, suffocating odor. It is a volatile organic compound, meaning it readily becomes a vapor or gas. It is also naturally produced in small, harmless amounts in the human body. The primary way we can be exposed to formaldehyde is by breathing air containing it. Releases of formaldehyde into the air occur from industries using or manufacturing formaldehyde, wood products (such as particle-board, plywood, and furniture), automobile exhaust, cigarette smoke, paints and varnishes, carpets, and permanent press fabrics. Nail polish and commercially applied floor finish emit formaldehyde.

In general, indoor environments have consistently higher concentrations than outdoor environments because many building materials, consumer products, and fabrics emit formaldehyde. Levels of formaldehyde measured in indoor air range from 0.02–4 parts per million (ppm). Formaldehyde levels in outdoor air range from 0.001 to 0.02 ppm in urban areas.

How Do You Know if a Household Product is Hazardous?

The FSHA requires that if a product contains a hazardous substance, it must bear a label of a specific size that contains certain information, depending on the product's toxicity.

“Signal Words” are found on every hazardous product label and indicate how toxic or hazardous a product could be (Table 2). If a signal word is not on a product, it is not hazardous.

Table 2. FSHA Signal Words for Household Products

Signal Words	Properties	Examples
POISON	highly toxic	varnish remover, antifreeze
DANGER	extremely flammable, corrosive, or highly toxic	bleach, WD-40
WARNING CAUTION	all other hazardous products	carpet cleaner, cleanser

Pesticides Have Additional Labeling

Pesticides are chemicals used to prevent, destroy, or repel pests, including insects, mice, weeds, fungi, and bacteria. Pesticides also include household products, such as disinfectants or cleaners, used to prevent the growth of harmful bacteria, viruses, or fungi on household surfaces.

The Environmental Protection Agency (EPA) registers and regulates pesticides under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). These products must meet the basic standards:

1. The product will not cause harmful effects to human health or the environment, and
2. Product labeling must meet FIFRA requirements. Examples: insecticides, rodenticides, herbicides, fungicides, microbial pesticides, etc.

Pesticides also have signal words, but they are based on the product's toxicity (Table 3). Tests are conducted to determine the Lethal Dose50 (LD50) of each pesticide.

LD50 is the dose at which 50% of a test population (usually mice or rats) dies when administered a specific dose of a pesticide. This is then calculated to a human dose.

The oral LD50 of a substance is expressed in milligrams of chemical per kilogram of body weight (mg/kg).

Table 3. FIFRA Signal Words and Toxicity Rating Scale for Pesticides

Signal Words	Toxicity	Oral LD50 (mg/kg)	Examples
DANGER-POISON	highly toxic	0 - 50	indoor/outdoor insect killer
DANGER	highly toxic/corrosive	0 - 50	toilet bowl cleaner
WARNING	moderately toxic	50 - 500	flea spray
CAUTION	slightly toxic	500 - 5,000	rat poison

Read Before You Use!

Besides signal words, product labels contain other important information, such as instructions for safe handling, use, and storage, active ingredients, and first aid safety.

As a consumer, make it a habit to read all label information before using any product.

Radiation

Radiation is energy given off by atoms and is all around us. We are exposed to radiation every day from natural sources like soil, rocks, and the sun. We are also exposed to radiation from man-made sources, such as medical X-rays and smoke detectors. We’re even exposed to low levels of radiation on cross-country flights, from watching television, and even from some construction materials. You cannot see, smell, or taste radiation. Some types of radioactive materials are more dangerous than others. So it’s important to carefully manage radiation and radioactive substances to protect health and the environment.

Radon is a naturally occurring, colorless, odorless radioactive gas. It comes from the natural decay of uranium or thorium, which are found in nearly all soils. It typically moves up through the ground and into the home through cracks in floors, walls, and foundations. It can also be released from building materials or from well water. Radon decays quickly, releasing radioactive particles. Long-term exposure to these particles can lead to lung cancer. Radon is the leading cause of lung cancer among nonsmokers, according to the U.S. Environmental Protection Agency, and the second leading cause behind smoking.