If you shower carefully with soap and shampoo, Karam says [Andrew Karam, radiation expert], the radioactive dust should wash right out. But hair conditioner has particular compounds called cationic surfactants and polymers. If radioactive particles have drifted underneath damaged scales of hair protein, these compounds can pull those scales down to create a smooth strand of hair. "That can trap particles of contamination inside of the scale," Karam says.

These conditioner compounds are also oily and have a positive charge on one end that will make them stick to negatively charged sections of a strand of hair, says Perry Romanowski, a cosmetics chemist who has developed personal hygiene formulas and now hosts "The Beauty Brains" podcast on cosmetics chemistry.

"Unlike shampoo, conditioners are meant to stay behind on your hair," Romanowski says. If the conditioner comes into contact with radioactive material, these sticky, oily compounds can gum radioactive dust into your hair, he says.