What fraction of hydrogen in air results in an explosive mixture?
Hydrogen is explosive in mixtures in air at room temperature and ambient pressure when it is between 4% and 75% by volume.  These limits are respectively called the "lower explosive limit" (LEL) and the upper explosive limit (UEL). 

Values for hydrogen (and many other gases) are provided in many locations on the internet.  Engineering toolbox is one such site.

More info on LEL and UEL is available from Wikipedia.