19-3 Chemical Hazards

What Are Toxic and Hazardous Chemicals? Agents of Death and Harm

A poison or toxin is a chemical that, through its action on life processes, can cause temporary or permanent harm or death to humans or animals. Its toxicity is often measured in terms of its median lethal dose. A hazardous chemical can harm humans or other animals because it is flammable or explosive or because it can irritate or damage the skin or lungs, interfere with oxygen uptake, or induce allergic reactions.

There are three major types of potentially toxic agents:

  1. One consists of mutagens, chemicals or ionizing radiation that cause or increase the frequency of random mutations, or changes, in the DNA molecules found in cells. Most mutations are harmless. One reason is that organisms have biochemical repair mechanisms that can correct mistakes or changes in the DNA code.

  2. A second type consists of teratogens, chemicals that cause harm or birth defects to a fetus or embryo. Ethyl alcohol is an example of a teratogen. Thalidomide is also a potent teratogen.

  3. The third group is carcinogens, chemicals or ionizing radiation that cause or promote cancer—the growth of a malignant (cancerous) tumour, in which certain cells multiply uncontrollably.

What Effects Can Some Chemicals Have on Immune, Nervous, and Endocrine Systems? Possible Harm from Small Doses

The immune system consists of specialized cells and tissues that protect the body against disease and harmful substances by forming antibodies that make invading agents harmless. Ionizing radiation and some chemicals can weaken the human immune system and leave the body vulnerable to attacks by allergens, infectious bacteria, viruses, and protozoans.

Some natural and synthetic chemicals in the environment, called neurotoxins, can harm the human nervous system that transmit electrochemical mes- sages throughout the body. Effects can include behavioural changes, paralysis, and death.

The endocrine system is a complex network of glands that release very small amounts of hormones in the blood- stream of humans and other vertebrate animals. Low levels of these chemical messengers turn on and off bodily systems that control sexual reproduction, growth, development, learning ability, and behaviour.

Case Study: Are Hormonally Active Agents a Human Health Threat? Serious Concern but Inconclusive Evidence

Hormonally active agents (HAAs) are synthetic chemicals that can mimic or disrupt natural hormones, affecting the endocrine system. Examples include DDT, PCBs, and certain herbicides. Some HAAs act as estrogen mimics, while others function as hormone blockers, interfering with sexual development and reproduction.

A method for testing hormonal activity in chemicals involves temperature-dependent sex determination (TDS) in red-eared slider turtles. Normally, eggs incubated at 25°C hatch as males, while those at 30°C hatch as females. However, exposure to hormonally active chemicals can override this pattern.

  • Experiment 1: Eggs incubated at 25°C and exposed to genistein (a soy-derived compound) showed feminization, with 30–70% hatching as females.

  • Experiment 2: Eggs incubated at 30°C and exposed to triclocarban (an antimicrobial agent) showed masculinization, with 88% hatching as males.

These findings raise concerns about the impact of HAAs on wildlife and humans. Documented effects include declining sperm counts, genital abnormalities, intersex conditions in fish and polar bears, and reproductive impairments in humans exposed to pesticides. The widespread presence of endocrine-disrupting chemicals threatens the hormonal balance of ecosystems and species, including humans.

Is Pollution Prevention the Answer? Taking Precautions

We lack comprehensive knowledge about the potential toxicity of many chemicals in our environment, and assessing their effects is costly and complex. To address this, some scientists and health officials, particularly in the European Union, advocate for pollution prevention—avoiding the release of harmful chemicals by finding safer alternatives or recycling hazardous substances before they reach the environment.

This strategy aligns with the precautionary principle, which states that when there is plausible but incomplete evidence of harm, precautionary action should be taken rather than waiting for full scientific certainty. This approach shifts the burden of proof onto those introducing new chemicals or technologies, requiring them to demonstrate safety before market approval.

Steps have been taken in this direction, such as the 2004 global treaty banning 12 persistent organic pollutants (POPs), including DDT and PCBs, with provisions to add more harmful chemicals over time. However, industries argue that strict application of the precautionary principle could hinder innovation and make it difficult to introduce new technologies and chemicals.

Proponents counter that the principle would drive innovation toward safer alternatives and pollution prevention rather than relying solely on pollution control. While there is no clear consensus on when and how to apply the precautionary principle, balancing innovation with risk reduction remains a key challenge in chemical regulation.