Eye irritancy tests, such as the LD50 test, are C’s on the USDA's pain scale. This means that they are neither painless nor painful, but in the middle, producing momentary pain or distress . With no certain species being targeted for this test, the animal used depends on the test performed. This test was originally designed to test both (1) how big a dose of something a human would have to intake to be killed and (2) to kill half of the test group being used in a certain experiment. This test was introduced by J. W. Trevan, a British pharmacologist. Eye irritancy tests were first started following World War I. Scientists wanted to study the soldiers’ prolonged exposure to mustard gas and the effects it had on them. Scientists first tested on rabbits’ eyes to compare the result they got with the results the soldiers were getting. In this test and many other situations, animals are given far above the normal human intake.