air
atmosphere
birds
blue
chimney
city
climate
cloud
construction
contaminant
contamination
damage
danger
dawn
defilement
dioxide
dirty
dirtying
ecology
electricity
emission
energy
environment
exhaust
factory
fuel
fume
gas
global
impurity
industry
manufacture
orange
pipe
plant
poison
pollution
power
sky
smog
smoke
soiling
station
steam
sun
sunrise
swirling
technology
toxic
vapor
warming