Atrazine is used as a selective preemergent herbicide on crops, including corn, sorghum, and sugar cane, and as a nonselective herbicide along fence lines, right-of-ways, and road sides. It is used in a variety of formulations, both alone and in combination with other herbicides. In the United States it is the single most widely used herbicide, accounting for about 15% of total herbicide application by weight. Its popularity stems from its effectiveness, its selectivity (it inhibits photosynthesis in plants lacking a detoxification mechanism), and its low mammalian toxicity (similar to that of table salt). Concern over its use stems from the fact that a small percentage of the amount applied can be carried by rainfall into surface waters (stream and lakes), where it may inhibit the growth of plants and algae or contaminate drinking-water supply. Because atrazine is classified as a possible human carcinogen, the United States Environmental Protection Agency (EPA) has limited its concentration in drinking water to 3 μg/L.