According to About.com, ppb stands for parts per billion and ppm stands for parts per million. To convert a ppb figure to a ppm figure, divide the ppb figure by 1,000.
Wikipedia notes that parts-per notation is used to describe very low concentrations in chemistry and for similar purposes in physics and engineering. It's a dimensionless number with no units. As fractions with round denominators, they're conceptually very similar to percentages.
Since a billion is a thousand millions, one part per billion is one-thousandth of a part per million. Conversely, one part per million is a thousand parts per billion. Thirty parts per billion would be 0.03 parts per million.