A recent study (Bakken et al, 2002) showed that over one million Americans visit a hospital each year as a result of falling on stairs.
Furthermore, 4,000 of those US citizens die each year as a result of those falls.
That number is about the same as the number of American pedestrians killed in collisions with vehicles.
It is twice the number of citizens killed in motor-cycle accidents.
Funnily enough (or ironically if you like), present day US building codes for stair risers (the vertical height of the stair) and depth of tread (the horizontally width of the stair) are based on a formula proposed by Frenchman, Francois Blondel in 1670.
Blondel based his formula on the stride-distance and foot-size of the average Frenchman living in the 1660s.
So US building codes for stairs are based on information that's almost 350 years old!
Even odder, for today's building codes, is that Blondel used a measurement known as "Royal Inches" which differ significantly from what we today consider to be the length of a modern inch.
So the fact that Frenchmen in the middle of the 1600s were a different size to modern Americans and the fact that the measurements used to define the ideal stair riser are so out-dated probably has something to do with all this harm!
Source: Bakken, Cohen, Hyde and Abele (2002). Slips, Trips and Mis-steps and their Consequences.