Neurotoxicity Definition
Neurotoxicity is defined as adverse effects on the structure or functioning of the central and/or peripheral nervous system that result from exposure to chemical substances. Neurotoxicants can cause morphological changes that lead to generalized damage to nerve cells (neuronopathy), injury to axons (axonopathy), or destruction of the myelin sheath (myelinopathy). It is well established that exposure to certain agricultural and industrial chemicals can damage the nervous system, resulting in neurological and behavioral dysfunction. Symptoms of neurotoxicity include muscle weakness, loss of sensation and motor control, tremors, alterations in cognition, and impaired functioning of the autonomic nervous system
The central nervous system (CNS) is composed of the brain and spinal cord. It is responsible for the higher functions of the nervous system (conditioned reflexes, learning, memory, judgment, and other functions of the mind). Chemicals toxic to the CNS can induce confusion, fatigue, irritability, and other behavioral changes. Methyl mercury and lead are known CNS toxicants. Exposure to these metals can also cause degenerative diseases of the brain (encephalopathy).
The peripheral nervous system (PNS) includes all the nerves not in the brain or spinal cord. These nerves carry sensory information and motor impulses. Damage to the nerve fibers of the PNS can disrupt communication between the CNS and the rest of the body. The organic solvents carbon disulfide, n- hexane,and trichloroethylene can harm the PNS, resulting in weakness in the lower limbs, prickling or tingling in the limbs (paresthesia), and loss of coordination.
Exposure to chemical agents can trigger a wide range of adverse effects on the nervous system. Neurotoxic substances can alter the propagation of nerve impulses or the activity of neurotransmitters and can disrupt the maintenance of the myelin sheath or the synthesis of protein. As a result, neurotoxicological assessments require the administration of a battery of functional and observational tests. Neurotoxicity in humans is most commonly measured by neurological tests that assess cognitive, sensory, and motor function.

Reproductive Toxicity Definition
Reproductive toxicity is defined as adverse effects on the male and female reproductive systems that result from exposure to chemical substances. Reproductive toxicity may be expressed as alterations in sexual behavior, decreases in fertility, or loss of the fetus during pregnancy. A reproductive toxicant may interfere with the sexual functioning or reproductive ability of exposed individuals from puberty throughout adulthood. Developmental toxicity (adverse effects on the developing child, such as birth defects) is often considered to be a subcategory of reproductive toxicity, but is treated as a distinct health endpoint in EDF's hazard identification system (see the separate definition of developmental toxicity).
Toxicants that target the female reproductive system can cause a wide variety of adverse effects. Changes in sexual behavior, onset of puberty, cyclicity, fertility, gestation time, pregnancy outcome, and lactation as well as premature menopause are among the potential manifestations of female reproductive toxicity: all can disrupt a woman's ability to successfully reproduce. Exposure to lead, for example, can result in menstrual disorders and infertility. The toxicants carbon disulfide, mercury, and polychlorinated biphenyls (PBCs) have been shown to cause irregularities in the menstrual cycle. An increased rate of spontaneous abortion has been found in hospital workers exposed to ethylene oxide.
Toxicants that target the male reproductive system can affect sperm count or shape, alter sexual behavior, and/or increase infertility. Carbon disulfide and the pesticides chlordecone (kepone), ethylene dibromide (EDB), and dibromochloropropane (DBCP) are examples of chemicals known to disrupt male reproductive health. These toxicants have lowered the sperm counts of exposed individuals. Occupational exposure to DBCP has caused azoospermia (the total absence of sperm in the semen) and sterility.

Respiratory Toxicity Definition
Respiratory toxicity is defined as adverse effects on the structure or functioning of the respiratory system that result from exposure to chemical substances. The respiratory tract consists of the nasal passages, pharynx, trachea, bronchi, and lungs. The chief function of the respiratory system is to ensure the efficient and effective exchange of oxygen and carbon dioxide between the blood and the air. Respiratory toxicants can produce a variety of acute and chronic pulmonary conditions, including local irritation, bronchitis, pulmonary edema, emphysema, and cancer.
It is well known that exposure to environmental and industrial chemicals can impair respiratory function. The air we breathe may contain toxic gases, vapors from solvents, aerosols, or particulate matter, all of which can cause respiratory harm. For example, scientific evidence has shown that ozone and fine particles pose a significant threat to respiratory health. Ground-level ozone, the main component in smog, causes breathing problems, aggravates asthma, and increases the severity and incidence of respiratory infections. More rarely, toxicants such as the herbicide paraquat may affect the respiratory tract after ingestion or dermal exposure.
Acute exposure to respiratory toxicants can trigger effects ranging from mild irritation to death by asphyxiation. Prolonged exposure to respiratory toxicants can cause structural damage to the lungs, resulting in chronic diseases such as pulmonary fibrosis, emphysema, and cancer. Pulmonary fibrosis is a serious lung disease in which airways become restricted or inflamed, leading to loss of elasticity and difficulty in breathing. It can be caused by exposure to coal dust, aluminum, beryllium, and carbides of tungsten. Emphysema, a degenerative and potentially fatal disease, is characterized by the inability of the lungs to fully expand and contract. The most common cause of emphysema is heavy cigarette smoking, but the disease can also be induced by exposure to aluminum, cadmium oxide, ozone, and nitrogen oxides. In addition, several toxicants are known to cause respiratory cancer. Examples of well-established human lung carcinogens are cigarette smoke, asbestos, arsenic, and nickel.

Skin or Sense Organ Toxicity Definition
Skin and sense organ toxicity is defined as adverse effects on the skin or sense organs that result from exposure to chemical substances. The senses of smell, vision, hearing, and taste are referred to as the special senses. Sense organs may be injured by a variety of physical, chemical, and biological agents. The sense of smell can become impaired, for example, as a result of occupational exposure to the metals cadmium and nickel. A variety of chemicals that act both locally and systemically in the body can affect vision. Airborne chemicals such as inorganic irritant gases and vapors (ammonia, chlorine, hydrogen sulfide, and sulfur dioxide) formaldehyde, and many organic solvents can cause external eye irritation, including conjunctivitis and keratitis, in some cases reducing visual acuity. Chemical substances can induce auditory dysfunction as well. Occupational exposure to lead, for example, is associated with hearing loss.
The skin provides the body with a protective barrier, contains the nerves that support the sense of touch, and participates in the exchange of gases and liquids. When the skin is exposed to irritant compounds, symptoms of skin injury may occur: redness, inflammation, burning, and itching. Acute and chronic skin diseases that may result from contact with toxic agents include dermatitis, photosensitization, chloracne, urticaria, and cancer.
One of the most common chemically induced skin disorders is contact dermatitis, an inflammatory reaction of the skin that is usually restricted to the exposed areas. Clinical signs may include reddening, edema, itching, or a burning sensation. Strongly alkaline and acidic substances such as sodium hydroxide, sulfuric acid, and hydrofluoric acid are some of the irritants that can cause contact dermatitis and related disorders. A number of skin toxicants produce photosensitization, an abnormal reaction to ultraviolet and/or visible radiation. Known photosensitizers include anthraquinone dyes, sulfanilamide, and coal tar derivatives (anthracene, pyridine, acridine, and phenanthrene). Chloracne is a severe and unusual form of acne that can be triggered by exposure to certain halogenated aromatic compounds, such as polychlorinated dibenzo furans and dioxins. Finally, arsenic, coal tars, creosote oils, and ultraviolet light have been shown to induce skin cancer in humans.