Question on Carbone monoxide and levels!!?

I am curious if carbon monoxide is dangerous at any level wether it shows 1 on the meter or 30. Because the other day we had a scare with our meter showing 9. We called the fire department and they told us to step outside, and asked if we were feeling ok ....They checked it and said we were ok and it was a dead battery setting it off...and last night it showed 11 and i figured it was the dead battery setting it off again so we called the maintence lady at our apartment and she said its not dangerous till it shows 30. But why would you chance it when its such a deadly cause? Is that true its not dangerous till it shows 30 parts per million??