I/50 resists 50 pounds dropped from 12 inches.I/75 resists 50 pounds dropped from 18 inches.
What do these figures mean? I/75 rated toe-caps must pass a foot-pound test by dropping a 50-pound steel weight from a height of 18 inches. This is measured in foot-pounds and will protect a worker from an impact of 75 foot-pounds.
There are two classifications for compression ratings--also Classes 50 and 75. The shoes are tested to withstand compression at a set poundage before the toe-cap begins to crack or collapse:

C/50 = 1,750 pounds
C/75 = 2,500 pounds
The ASTM standard F2413-05 dropped a previous Class 30 rating from both impact and compression ratings. The standard also excludes "hang-on" or "strap-on" toe-caps, which are added on to the shoe. The toe-cap must be an integral, built-in part of the footwear. Also, the standard has removed static dissipative (SD) type II shoes, leaving only type I.
Read the Label
Here is a sample of a label you might find in safety footwear:
ASTM F2413-05
M I/75 C/75
PR
That code breaks down in this manner:
ASTM F2413-05 is the ASTM standard.
M I/75 C/75:
M or F identifies the gender (M = Male or F = Female)
I/75 is the impact resistance rating.
C/75 is the compression resistance rating.
That list of ASTM initials reminds us that feet in the workplace can be injured in a great variety of ways, so many factors need to be taken into consideration. Slip resistance, for example, is a factor employers need to ponder. Are employees in danger of slipping on slick surfaces? OSHA requirements for fire brigades, in 1910.156(e)(2)(ii), call for Class 75 footwear that is water-resistant for at least 5inches above the bottom of the heel and equipped with slip-resistant outer soles.
Written By: Mike Girolami, President of Michigan Industrial Shoe http://wwwmichiganshoe.com/, Local Distributor of Work Boots, Safety Shoes and Work clothing, Redford MI