Out of roughly 100,100 chemicals currently in commercial use, some man-made chemicals have been proven to be hazardous to human health and the environment. These synthetic chemicals have undesirable properties such as low biodegradability, which means that substances break down very slowly in the environment. Another characteristic is the high bioaccumulation potential, which has a tendency to accumulate in food chains. 

Two of the most worrying aspects of the effects of toxic chemicals are biomagnification and health impacts on the young, who are most vulnerable. Certain chemical substances are highly stable in nature. As a result, they have long-lasting effects and affect large areas before being broken down into a harmless form. The risk of a stable compound is that it is capable of bioaccumulation in fatty tissues of living organisms in concentrations many times higher than in the surrounding environment. Predators may accumulate certain chemicals in even higher concentrations (known as biomagnification), with mammals, including humans, topping the list because of meat and fish consumption.

Exposure to toxic chemicals may create particularly significant adverse health effects during early stages of the lifecycle for foetuses in the womb or during childhood when the endocrine system is actively built. Even with small dose exposures, the consequences can in some instances be devastating with problems ranging from serious impacts on human development, chronic diseases, and learning disabilities.
 
Typical environmental and human health criteria that are used to identify the degree of toxicity of certain chemicals include:

Environmental hazards

• Algae toxicity
• Bioaccumulation
• Climatic relevance including ozone-depleting potential
• Content of halogenated organic compounds
• Daphnia toxicity
• Fish toxicity
• Heavy metal content
• Persistence and biodegradation
• Water danger and toxicity to soil organisms

Human health hazards

• Carcinogenicity
• Mutagenicity
• Reproductive toxicity
• Teratogenicity (or the potential for fetal malformation)
• Endocrine disruption
• Acute toxicity (the adverse effects resulting from a single exposure or a substance)
• Chronic toxicity (the adverse effects from repeated exposures to a single substance over a longer time period)
• Irritation of skin and mucous membranes
• Sensitization

Based on the above criteria, hazardous chemicals that are the focus of international action for ultimate phase-out are classified as follows:

Persistent organic pollutants (POPs)
These are chemical substances that persist in the environment, bioaccumulate through the food chain, can be transported long distance to regions, and pose a risk of causing adverse effects to human health and the environment. The 12 most harmful substances that are targets for global action banning usage:

• Dioxins (PCDDs), which are a set of unintentional chemical by-products,
• Two industrial materials (Polychlorinated Biphenyls (PCBs) and Hexachlorobenzene (HCB)), and
• Nine pesticides (DDT, Aldrin, Dieldrin, Endrin, Chlordane, Heptachlor, Mirex, Toxaphene, and Furans (PCDFs)).

Endocrine disruptor chemicals (EDCs)
EDCs are chemical substances that interfere with the functioning of the endocrine system. Most of the information on EDC exposure has focused on the presence of POPs such as PCBs, dioxins, DDT, other chlorine-containing pesticides, bisphenol-A, and phthalates.

The endocrine system is responsible for releasing hormones and chemical messages which help guide development, growth, reproduction, and behaviour of animals and humans. The presence of EDCs in our environment raises concern because of:

1. Harmful effects that have been observed on reproduction, growth and development in certain species of wildlife, and
2. Observed increases in some human reproductive disorders and cancers which could be related to disturbance of the endocrine system.

Despite the lack of comprehensive information on how EDCs act, what is known at present is that exposure to EDCs during the early development of the endocrine system for foetuses or during childhood may permanently alter the function or reactions of the system to various signals in the body.

Carcinogens, mutagens, and reproductive toxicants (CMRs)

• Carcinogens - substances that cause cancer. The International Agency for Research on Cancer (IARC) has classified some 35 substances as probably or definitely carcinogenic in humans such as benzene, cadmium, ethylene oxide, and vinyl chloride
• Mutagens - substances that are capable of causing a gene-change or DNA damage. As many mutations cause cancer, mutagens are typically also carcinogens
• Reproductive toxicants - substances that cause adverse effects on the male and female reproductive systems such as alterations in sexual behavior, decreases in fertility and foetus mortality during pregnancy. Lead, carbon disulfide, mercury, PBCs, chlordecone, ethylene dibromide (EDB), and dibromochloropropane (DBCP) are examples of toxicants.

These are the main websites that provide lists of toxic chemical substances and basic information on toxicity.