Four ways in

A joiner cuts old boards all day in a shed with the door shut. The air is grey. Nothing hurts. Twenty years on, the lungs that breathed that air are scarred, and the shed is long gone. A substance does its harm by getting into the body, and most of the harm that takes years gets in this way.

There are four ways in. Breathed in, through the nose and mouth to the lungs. Swallowed, usually from hands that touched it and then touched food. Through the skin, which some liquids pass straight through. And through a cut, a splinter or a needle, straight into the blood. Breathing is the route that carries most of the harm.

Paint is sprayed in a closed garage with no fan. Tap the route most of it takes into the painter.

Tap the part of the picture that answers it.

How much harm depends on the dose, and the dose is simple: how much is in the air, times how long you breathe it. Britain sets limits for many substances as an average over an eight-hour day. That gives two levers. Lower the level, with a fan or a safer substance. Or shorten the time.

Move the control to set how long a worker spends in the dust. Picture dust at 2 units in the air, and a day's limit set for an eight-hour shift, so the line sits at 16. Made-up round numbers.

Move the control.

The worst case is asbestos, a fireproof mineral built into millions of buildings until it was banned in Britain in 1999. Its fibres, breathed in, cause cancers that appear decades later, often 20 to 50 years on. Around 5,000 people a year still die from it in Britain. The cause was in the air before many of them were born.

For every worker killed in an accident in Britain in a year, about how many people die from asbestos they breathed in decades ago?

Pick one. Nothing is scored.

• About 1
• About 5
• About 40
• About 400

Each card is a substance meeting a worker. Which way in?

One card at a time. Tap the pile it belongs to.

A garage mechanic washes oily hands in petrol every evening. Which route is that, and which answer would the ladder of controls reach for first?

• Through the skin. Swap the petrol for a proper hand cleaner, so the hazard itself is gone.
• Through the skin. Give the mechanic thick gloves to wash in.
• Breathed in. Fit a fan over the sink.