This site from the Environmental Protection Agency has links to sites on a variety of environmental health topics. Some sources are at the middle or high school level but many are good for Kindergarten on up.
Allows you to explore "environmental health concerns and toxic chemicals where you live, work, and play." Locations include a farm, a port, a city, a town, and the Southwest. Curriculum units and text and Spanish-language versions are also available in the "For Educators" section.
"What's under your kitchen sink, in your garage, in your bathroom, and on the shelves in your laundry room? Learn more about what's in these products, about potential health effects, and about safety and handling."