M3 · Toxicology & Routes of Exposure

Module Three. Toxicology and Routes of Exposure. Recognition tells you a hazard is there. This module is about what it does once it gets in. Module One taught you to recognise a hazard. Module Two taught you to assess a site. This module is about the part you cannot see. What a hazard does once it gets into your body, and how it gets there. Here is why it exists. The same material can be harmless one day and harmful another. What changes is the amount, how long you were exposed, and how it got in. In the next few minutes we will cover four things. The four ways a hazard enters the body. Acute and chronic effects, and why timing matters. What our hazards attack, hazard by hazard. And exposure limits, with what a meter can and cannot tell you.

The invisible part. The hazard you cannot see. A hazard can be present with no smell, no colour, and no immediate effect. That is what makes exposure easy to underestimate. Recognition answers the question, is it there. Toxicology answers a different one. What does it do, and how much does it take. Most of the harm on our work is not dramatic. It is cumulative, and it is easy to miss while it is happening. Three things work against you. No warning. Odourless gases, and hazards that deaden your sense of smell. No symptoms yet. Chronic effects show up years later, not on the day. And small doses add up. Dose is amount, time, and frequency, not one event. This is why the controls exist even when the job looks fine. Feeling fine is not evidence of anything.

How it gets in. Four routes of entry. Every exposure on our work uses one of four doors. Inhalation. Breathing vapour, mist, dust, or gas. Usually the fastest route. Absorption. Through skin or eyes. No cut needed. Some chemicals pass straight through. Ingestion. Hand to mouth. Eating, drinking, smoking, or a contaminated glove. And injection. Through a break in the skin. Needlesticks, cuts, a sharp from the debris. Inhalation and absorption are the ones that catch crews on our jobs. Injection is the one that changes your life.

In the field. How each route happens here. Not theory. These are the four ways it actually happens on our work. Inhalation. Solvent vapour in a closed room. Dust when cutting. Gases from decay or sewage. Absorption. Solvent straight through a glove. A splash to the eye. Handling soaked material. Ingestion. Eating or smoking with contaminated hands. A glove you touched your face with. And injection. A needlestick from loose sharps. A cut on glass or metal in the pile. The route decides the protection. A respirator does nothing for a splash. Gloves do nothing for vapour.

Dose. Dose makes the poison. How much, how long, how often. Change any one of those and the outcome changes. Two crews can meet the same material and get different outcomes. The difference is the dose. How much. The concentration in the air or on the surface. How long. Minutes of exposure, or a whole shift. And how often. Once, or every day for a year. Concentration times time. A short, high exposure and a long, low exposure can both hurt. The body does not care which one it was. It was only a little is only true if the other two numbers are small as well.

Timing. Acute and chronic. Some effects land immediately. Others take years to show, and by then the damage is done. Acute effects come fast, from a short high exposure. Dizziness, burns, breathing trouble. Hydrogen sulfide is the classic one on our work. Chronic effects build over months or years. Asbestosis, silicosis, lead effects, some cancers. No warning on the day. Same hazard, two timelines. Carbon monoxide can drop you in minutes. Asbestos can take thirty years. Both are real, and both are managed the same way. Keep it out of your body.

Target organs. What our hazards attack. Name the hazard, know what it attacks. That is what the controls are protecting. Asbestos attacks the lungs and chest lining, chronic. Crystalline silica attacks the lungs, and at higher exposure the kidneys and immune system, chronic. Lead paint attacks the nervous system, kidneys, and blood, chronic. Solvents and fuels attack the brain and nerves, liver, and kidneys, both acute and chronic. Carbon monoxide attacks the blood and blocks oxygen, acute. Hydrogen sulfide attacks breathing and the nervous system, acute. Bloodborne pathogens attack the liver and immune system, by infection. Sewage and feces cause gut infection, tetanus, and hepatitis A. Mould attacks the airways, and it sensitises. Ammonia attacks the airways, eyes, and skin, acute. And oxidisers, for example peroxide, attack the skin and eyes, acute. The route and the target together tell you what the protection has to do. Lungs need a respirator. Skin needs the right glove. Eyes need a shield.

The numbers. Exposure limits in plain language. P E L, T L V, S T E L, I D L H. Four letter sets, and one idea. There is a level above which the risk climbs. P E L is the legal limit. The permissible exposure limit, averaged over a work shift. T L V is the recommended limit. A professional guideline, often tighter than the P E L. S T E L is a short-term limit. The ceiling for a brief exposure, usually fifteen minutes. And I D L H. Immediately dangerous to life or health. Above this you do not go in without serious protection, and usually you do not go in at all. Limits are set for a healthy working adult over a working lifetime. They are not a safety guarantee. They do not account for your medical history, mixed exposures, or a hot day. You do not need to memorise the numbers. You need to know a limit exists, that lower is better, and that the limit is not the target.

Instruments. What a meter can and cannot tell you. A meter reads the air at one spot, at one moment. That is useful, and it is limited. What it can do. Tell you the oxygen, flammable, and common toxic levels where you are standing, right now. What it cannot do. Tell you what is behind the next wall, what you cannot smell, or what a chemical does through skin. It is only as good as its calibration. A meter out of calibration is a false sense of safety. And it is not a control. A meter tells you a hazard is there. It does not remove it. Test order stays the same. Oxygen first, then flammables, then toxics. A reading below the limit is not permission to skip protective equipment.

Skin. The route people forget. Skin is not a barrier. It is a route. Solvents, some pesticides, and many industrial chemicals pass straight through intact skin. There is no splash, no cut, and no warning. A glove of the wrong type for the chemical can hold it against your skin and make it worse. And then there is sensitisation. Some people become sensitised after exposure. After that, even a tiny amount triggers a reaction, and it does not wear off. That is a permanent change, and it is why gloves and barriers have to match the chemical and the job. Soak-through, splash, and contaminated clothing all count. Change gloves when they are soiled, not at the end of the shift.

If it happens. Symptoms and the exposure rule. If something feels wrong, that is already information. Do not work through it. Early signs to act on immediately are headache, dizziness, nausea, eye or throat irritation, unusual tiredness, a rash, or simply feeling off. The exposure rule. Stop work and move to fresh air or a safe area. Tell your supervisor immediately. Not at break, not at the end of the day. Get the material identified, and get a medical evaluation. And do not wait and see. Effects that start small can get worse. Report even without symptoms if you took a hit. A splash, a needlestick, breathing vapour. The record matters, and so does the medical follow-up.

Recap. Five points to carry onto the next job. One. Four routes. Inhalation, absorption, ingestion, injection. Two. Dose is amount, duration, and frequency. Concentration times time. Three. Acute hits fast. Chronic builds for years and gives no warning on the day. Four. Exposure limits are thresholds, not guarantees. Lower is always better. Five. Anything that feels wrong, and any exposure at all, gets reported immediately. Next module. P P E and Respiratory Protection. Matching the protection to the hazard this module just described.