What is the lowest level of toxicity?

According to the Toxicology categories established by the U.S. Environmental Protection Agency, the lowest level of toxicity is Toxicity category IV, which is ‘Practically Non-Toxic and Not An Irritant’. The other three toxicology categories in descending order are Highly Toxic and Severely Irritating, Moderately Toxic and Moderately Irritating, and Slightly Toxic and Slightly Irritating. 

In terms of value, LD50 1,000 - 2,000 mg/kg indicates low toxicity levels.