Because ethylene dibromide, a chemical used to fumigate grain, was blamed for the high rate of nerve damage suffered by people who work in grain-processing plants, many such plants switched to other chemical fumigants two years ago  Since then, however, the percentage of workers at these plants who were newly diagnosed with nerve damage has not dropped significantly
Ttherefore, either ethylene dibromide was wrongly blamed or else the new chemicals also cause nerve damage
If ethylene dibromide causes nerve damage, it does not take two years or longer for that damage to become detectable.