air
animal
bird
birds
black
blue
cable
chemical
chimney
electric
electricity
energy
environment
environmental
exhaust
factory
flock
fuel
gas
global
group
industrial
industry
line
migration
nature
oil
plant
pollution
power
refinery
rest
resting
silhouette
sky
smoke
smokestack
spring
stack
summer
swallow
technology
telephone
tower
toxic
warming
white
wild
wildlife
wire