Floors were redone in apt below mine with polyurethane. smell came into my place and was very strong. they said it was safe for me to stay. was it?

General guidelines: suggest that those who might be especially affected by fumes--pregnant woman, elderly and sick and young children--should avoid exposure. Brief exposure such as you describe might cause some lung irritation or aggravation of asthma (if you have it) or a headache, but this brief exposure should not have worrisome long term health effects for you. Stay well:)