All in the dose

Poisons are substances that can be lethal. They are chemicals, either manufactured or naturally produced. We know that everything is made from chemicals, so are all chemicals poisons? Actually, it depends on a number of things – most importantly the amount of the substance absorbed, inhaled or ingested.

Over 400 years ago, Swiss alchemist and physician Paracelsus (1493–1541) said:

All substances are poisons; there is none that is not a poison. The right dose differentiates a poison from a remedy.

The principle that the dose makes the poison is paramount. It means that all chemicals, from whatever source – human manufacture or natural – are potentially toxic at some dose. This fundamental concept underlies toxicology and is crucial to the assessment of risk from chemicals and their safe use. It is known as the dose-response relationship – it is the amount of the substance that causes it to be toxic, not the substance itself.

Dose-response relationship

Some years ago, a death occurred due to the overconsumption of carrots. People have died from drinking too much water (water intoxication). Too much oxygen (vital for life) results in cell damage and can lead to death (oxygen toxicity). Likewise, even for extremely toxic substances such as snake venom, there is a dose below which there is no detectable toxic effect.

The dose-response relationship is fundamental in determining safe concentrations. The dose is the total amount of a substance administered to, taken or absorbed by an organism.

Here are some examples of dose-related toxicity:

• Ingestion of common salt (sodium chloride) is essential for human health in small doses but may be harmful in large doses. It can cause high blood pressure and can lead to strokes or cardiovascular disease.
• Paracetamol is commonly used for pain relief and to reduce fever in small doses, but in larger doses, it causes liver damage and can lead to death.
• Alcohol is a psychoactive drug that has a depressant effect. It is often consumed because of its calming effects, and it can make people feel more sociable. In large doses, alcohol is harmful and can cause acute alcohol poisoning. This occurs when the liver is unable to effectively process all the alcohol you consume. As a result, your heart rate and blood pressure can lower. Your central nervous system can be affected, and your vital functions slow down so that you are unable to perform. In worst cases, it can lead to death. Long-term exposure to alcohol can lead to liver damage and ultimately death.

Factors affecting toxicity

Toxicity is the degree to which a substance can damage an organism, a tissue or a cell. It may be affected by a number of factors:

• Dosage – the amount of substance taken.
• Dose-time relationship. This is the combination of how much and how often. For example, exposure might be to a large dose of a substance, but it might occur once or it may be in small quantities but over a long period of time.
• Exposure – repeated exposure over time leads to toxicity, for example, lead poisoning.
• Exposure route – whether inhaled, ingested or absorbed through the skin.
• The species of animal being affected.
• Sex/gender, size and age of the affected animal or human.
• Chemical activity within the organism.
• The ability of a substance to be absorbed, distributed, metabolised and finally excreted from the body.
• The presence of other substances, for example, alcohol.

Uses for toxins and toxic substances

Some toxins and toxic substances in small doses can be beneficial. Toxins often occur naturally in food, but in small doses, they are fine and even beneficial. Certain toxins in small doses are used as medications to heal us when we are sick. Other toxic substances are used as sweeteners, preservatives, antiseptics, perfumes, dyes and cleaning agents. In agriculture, they are used as fertilisers and pesticides to increase crop production.

Although useful, all potentially toxic substances should be handled correctly and used for the correct purposes – and not assumed to be harmless.