9781422276860

■ ■ A heating and cooling technician repairs wiring on an air-conditioner.

Words to Understand

allergen: Any substance such as pollen, dust, mold, and pet dander that is recognized by the immune system and causes an allergic reaction. aqueduct: A system of typically downward sloping pipes and support structures that brings water from a distant source to a city or other destination. geothermal power: Energy that is created by harvesting heat that exists below the earth. perishable goods: Food (fruits, vegetables, fresh meat, etc.) and medicines that deteriorate quickly if not refrigerated or frozen. pollutant: Any substance (such as tobacco and other types of smoke, household chemicals, etc.) that is introduced into the environment that causes a negative effect on humans, plants, animals, ecosystems, and the overall envi- ronment. solar energy: Energy that is produced by harvesting the heat and energy of the sun.