Factory managers in the 1920s encouraged female workers to lick paintbrushes coated in radioactive radium to keep the tips pointed while painting glowing numbers on watch dials.

A tiny brush stroke created a repeated exposure

The glowing numbers on watch dials had to be small and sharply defined. Painters used fine brushes to apply radium-containing paint, but the bristles quickly spread apart. Workers were encouraged to draw the brush between their lips to restore its point—a technique known as lip-pointing. That made each correction a possible route for swallowing paint dust or residue.

The practice turned a task repeated throughout the workday into repeated internal exposure. The women were told the material was safe, so the brush’s contact with their mouths did not seem like an obvious hazard.

Why the harm showed up in bones

Once swallowed, radium could enter the bloodstream. The body handles radium in some ways like calcium, a mineral used to build bones and teeth, so it can become lodged in bone tissue rather than simply passing through. There, its radiation can damage nearby cells over time.

That helps explain the delay between exposure and visible illness. A worker might feel well while radioactive material remained in her body; damage could accumulate before pain, dental problems, or injury to the jaw became apparent. The jaw was especially vulnerable because it contains bone and is affected by the surrounding tissues and teeth.

The danger was built into the work routine

The hazard did not come from the dial’s glow itself. It came from handling radioactive paint and repeatedly putting a paint-coated brush to the mouth. A technique that solved a small production problem—keeping a brush tip neat—also bypassed the barrier that would have kept the material out of a worker’s body.

That is what made the episode so consequential: a seemingly minor workplace habit could create an exposure that remained hidden long after the shift ended.