Izingxenye ezikhethekile noma imishini eyaziwa ngokuthi "izesekeli" zobufakazi "zenziwa ukunciphisa noma ukuqeda amathuba okuqhuma ezimweni eziyingozi. Ezindaweni zezimboni lapho ama-gasses avuthayo, ama-vasses avuthayo, umusi, uthuli, noma ezinye izinto ezivuthayo kungenzeka zibe khona, lezi zesekeli zisetshenziswa kaningi.