Environment Vocabulary N-W N natural gas 61 percent of US households used natural gas for heating in 2002. O oil slick Large oil slicks were found near the coast. overfertilization Over Fertilization is a common problem. oxygen The cells need oxygen to survive. ozone layer The ozone laye r screens out the sun's harmful ultraviolet radiation. P permission Ask permission for the privilege to cross private land. poison The cat was killed by rat poison . poisonous She has lots of poisonous plants in her garden. the polar ice caps If the polar ice caps melt, the ocean levels rise. to pollute Please do not pollute the air. ...