Chemicals and Human Health: Weighing Risks and Benefits

"Chemical-free"?

Some products are advertised as "chemical-free". A chemist reading that label smiles, because everything is made of chemicals: water, vitamins, shampoo, apples and you. The useful question is not "Does it contain chemicals?" but "Which chemicals, how much, and what do they do?"

In this lesson you will evaluate the benefits and risks to human health of common chemical substances: food additives, medicines and cosmetics. This is expectation B1.2, and it uses ideas you already know (ions, molecules, naming) to make better everyday decisions.

The dose makes the poison

In the 1500s the Swiss physician Paracelsus wrote, in effect, that all substances are poisons; only the dose decides. Modern toxicology still begins there.

• Dose is the amount of a substance taken in, often expressed per kilogram of body mass (mg/kg).
• Many substances are helpful at one dose and harmful at another. Vitamin A, iron, caffeine and table salt are all needed or useful in small amounts and harmful in large ones.
• LD₅₀ (lethal dose, 50%) is a laboratory measure of acute toxicity: the dose, in mg per kg of body mass, that kills half of a group of test animals. A smaller LD₅₀ means a more toxic substance. LD₅₀ does not describe long-term effects, so it is only one piece of evidence.

Two other key ideas:

• Hazard is the potential of a substance to cause harm.
• Risk is the chance that harm will actually happen, which depends on the hazard and the exposure (how much, how often and by what route people come into contact with it).

Food additives

Food additives are substances added to food for a purpose: to preserve it, add nutrients, improve texture, or add colour or flavour. In Canada, Health Canada must approve each food additive and sets limits on how much can be used.

• Sodium nitrite (NaNO₂) in cured meats such as ham and bacon keeps the pink colour and, importantly, prevents the growth of the bacteria that cause botulism, a deadly food poisoning. The risk: under very high heat, nitrites can form small amounts of nitrosamines, which are linked to cancer. Regulations limit the amount of nitrite used.
• Iodized salt contains potassium iodide (KI), added to prevent iodine deficiency, which can cause thyroid problems (goitre).
• Sodium chloride itself: Canadians on average eat more sodium than recommended. For people aged 14 and older, the recommended limit for reducing chronic disease risk is 2300 mg of sodium per day. Too much sodium is linked to high blood pressure.
• Caffeine (a molecular compound) is a stimulant. Health Canada advises that teens consume no more than about 2.5 mg per kg of body mass per day (adults up to 400 mg per day).

Medicines

Medicines (pharmaceuticals) are chemicals with large benefits and real risks. Every medicine has a therapeutic dose (the dose that helps) and a higher dose that causes harm.

• Acetaminophen relieves pain and fever. Taken above the label dose, it can cause serious liver damage. Because it is found in many cold and flu products, people can take too much by accident when they combine products.
• ASA (acetylsalicylic acid, sold as Aspirin) relieves pain and reduces blood clotting, but it should not be given to children or teens with viral illnesses such as flu or chickenpox, because it is linked to a rare, serious condition called Reye's syndrome.
• Antacids contain bases such as calcium carbonate or magnesium hydroxide that neutralize excess stomach acid.

Reading the label, using the right dose, and asking a pharmacist about combinations are how people get the benefits while limiting the risks.

A Canadian success story: in 1921 and 1922 at the University of Toronto, Frederick Banting, Charles Best, J.J.R. Macleod and the biochemist James Collip discovered and purified insulin. Collip's chemistry made the extract pure enough to treat patients. Insulin has since saved millions of lives of people with diabetes.

Cosmetics and personal care products

Shampoos, lotions, makeup and toothpaste are chemical mixtures. In Canada they must meet the Cosmetic Regulations, and Health Canada keeps a list of ingredients that are restricted or prohibited in cosmetics (the Cosmetic Ingredient Hotlist).

• Fluoride (sodium fluoride or tin(II) fluoride) in toothpaste strengthens tooth enamel and reduces cavities. Too much fluoride swallowed by young children over time can cause white spots on teeth (dental fluorosis), which is why children should use only a small amount of toothpaste and spit it out.
• Zinc oxide and titanium dioxide in sunscreens reflect and absorb ultraviolet light, reducing the risk of sunburn and skin cancer.
• Heavy metals: Health Canada has warned about some imported skin-lightening creams and cosmetics found to contain mercury or lead, which can be absorbed and cause serious health effects. Buying products sold legally in Canada, with full ingredient lists, reduces this risk.