A growing human population releases chemicals that never existed before the twentieth century, and some of them come back to us through our food and water. Most do little harm, because the body breaks them down and excretes them. A few are dangerous at astonishingly low levels, either because they fit a hormone receptor or because they persist and become more concentrated as they move up a food chain. Understanding which chemicals behave this way, and why, is what allows scientists and governments to set limits, ban products and protect the communities most exposed. This lesson connects your knowledge of hormones, the nervous system and food chains to the health of people and ecosystems.
Endocrine disruptors: chemicals that fit hormone receptors
An endocrine disruptor mimics, blocks or interferes with a hormone. Because hormones act at nanomolar concentrations and are amplified by their target cells, a disruptor can act at doses far below those that are toxic in the usual sense. The strongest Canadian evidence comes from the Experimental Lakes Area (ELA), a set of small lakes near Kenora in northwestern Ontario reserved for whole-ecosystem experiments since 1968. As described in the endocrine lesson, researchers led by Karen Kidd added ethinylestradiol (EE2), the synthetic estrogen in birth control pills, to Lake 260 for three summers (2001 to 2003) at about 5 to 6 nanograms per litre, similar to levels found downstream of some sewage plants. Male fathead minnows began making vitellogenin, an egg-yolk protein normally made only by females, and developed egg cells in their testes. Reproduction failed, and within two years the lake's minnow population had collapsed to near extinction. After the additions stopped, the population recovered. A whole-lake design mattered: laboratory tanks cannot show a population crash that unfolds over generations in a real food web, while untreated reference lakes showed the collapse was not caused by weather or disease.
| Contaminant | Where it comes from | How it disrupts | Canadian action |
|---|---|---|---|
| EE2 (synthetic estrogen) | Excreted in urine; passes through many wastewater plants | Binds estrogen receptors; feminizes male fish | Better wastewater treatment removes much of it |
| BPA (bisphenol A) | Polycarbonate plastics and can linings; leaches more when heated or scratched | Weakly mimics estrogen | In 2010 Canada became the first country to declare BPA toxic and banned polycarbonate baby bottles containing it |
| Phthalates | Plasticizers that make PVC (vinyl) soft; some cosmetics | Interfere with testosterone during fetal development (anti-androgenic) | Six phthalates restricted in soft vinyl children's toys and child-care products since 2011 |
| PFAS ("forever chemicals") | Non-stick and stain-resistant coatings, grease-proof packaging, firefighting foam | Bind blood proteins; linked to thyroid disruption, raised cholesterol and weaker vaccine responses | PFOS prohibited; Health Canada set a drinking-water objective of 30 ng/L for total PFAS in 2024 |
Bioaccumulation and biomagnification
- Build-up of a substance within one organism over its lifetime
- Happens when intake is faster than breakdown and excretion
- Older, larger individuals carry more
- Example: lead stored in human bone for decades
- Increase in concentration from one trophic level to the next
- Happens because each predator eats many prey and keeps their contaminant
- Top predators carry the most
- Example: methylmercury in walleye, DDT in fish-eating birds
Biomagnification needs three properties. The substance must be persistent (not broken down by enzymes or light), retained in tissue (dissolved in fat, like DDT and PCBs, or bound to proteins, like methylmercury and PFAS) and slowly excreted. Then the energy pyramid does the rest. Only about 10% of the energy at one trophic level reaches the next, so a predator must eat roughly ten times its own mass in prey. The energy is used up in respiration, but the contaminant stays. A widely cited summary of 1960s measurements in a Long Island estuary gives DDT at roughly 0.000003 ppm in the water, 0.04 ppm in zooplankton, 0.5 ppm in small fish, 2 ppm in larger fish and about 25 ppm in fish-eating birds. In those birds DDT's breakdown product interfered with calcium deposition in eggshells, which thinned and broke, and peregrine falcon and bald eagle populations crashed until DDT was restricted in the 1970s.
Mercury, Grassy Narrows and lead
From 1962 to 1970, a chlor-alkali plant serving the pulp and paper mill in Dryden, Ontario, released roughly 9 to 10 tonnes of mercury into the English-Wabigoon River. Downstream live the Anishinaabe communities of Grassy Narrows First Nation (Asubpeeschoseewagong) and Wabaseemoong (Whitedog), whose diet, culture and livelihoods centred on the river's fish. Bacteria in the river sediments convert inorganic mercury into methylmercury, which is absorbed easily, binds to proteins in fish muscle and biomagnifies, reaching the highest levels in walleye and other predators. Methylmercury crosses the blood-brain barrier and the placenta and kills neurons, causing the symptoms of Minamata disease, named for a Japanese poisoning: numbness in the hands and feet, tunnel vision, tremors, loss of coordination and slurred speech. The commercial fishery closed in 1970, taking jobs and a way of life with it, and many residents still live with symptoms. Because mercury remains in the sediments, fish are still contaminated more than 50 years later. Ontario committed funding in 2017 to study and clean up the river, and in 2020 the federal government agreed to fund a care home in the community for people with mercury poisoning; community members continue to press for full remediation and compensation.
| Feature | Methylmercury | Lead |
|---|---|---|
| Main sources | Predatory fish from contaminated water | Old paint, lead service pipes in older homes, historic leaded gasoline (phased out in Canada by 1990) |
| How it harms | Binds sulfur groups in proteins, damaging neurons | Mimics calcium, disrupting enzymes and nerve signalling; stored in bone |
| Most vulnerable | Fetuses and young children (developing nervous system) | Young children: lowers IQ and attention; no known safe level |
| Food-chain behaviour | Strongly biomagnifies | Bioaccumulates in each organism, but does not biomagnify much in food webs |
| Canadian limit | 0.5 ppm in most commercial fish | 5 µg/L maximum in drinking water (2019) |
Dose-response: the dose makes the poison
Toxicologists test a chemical at several doses and plot the response, for example the percentage of test animals affected. Most curves are S-shaped: below a threshold there is no measurable effect, the highest dose with no observed adverse effect is the NOAEL, and the dose that kills half the test population is the LD₅₀. Regulators divide the NOAEL by safety factors (often 10 for differences between species and 10 more for differences among people) to set a tolerable daily intake. Endocrine disruptors complicate this: some show effects at low doses that disappear at higher ones, and timing matters as much as dose, because a brief exposure during fetal development can permanently change how organs form. This is why limits for pregnant people and children are often stricter.