It depends on what is used, when, where, and how much is used.
Some chemicals are more dangerous than others.
If the applicators put up a sign warning to keep children and pets off the lawn for 24 hours, it is a good idea to stay away. But even relatively harmless lawn chemicals like fertilizers can have detrimental environmental effects if used in sensitive areas. Fertilizer can run off lawn when it rains and find their way into rivers and lakes. The fertilizer can cause an overgrowth of algae and water weeds, which can choke out fish and other animals. P.O. Box 100, Pueblo, CO 81002.