Koffing and Grimer, for all their difference in shape, are extremely similar pokemon. Both of them are of the poison-type, both the same shade of grayish purple, but not the poison of assassinations or a protection from predators, but the poison of modern industrial pollution. Some have even theorized that Koffing is merely the gaseous form of Grimer, but this has since been disproven: those few unfortunate Grimer who have faced fatally hot temparatures have merely broken up into their component gases when they melted.
These two pokemon are the source of the two major pollution problems facing urban environmental policy in today's pokemon world, but the problems are not identical. While Grimer are moving poisonous slime, Koffing possess a tough shell in which their toxic gases are stored. Although those who get too close are admittedly in danger, a city can contain a large Koffing population and still have the air regarded as safe to breathe for most of the year – indeed, Koffing shells are so stable that attempts to weaponize the pokemon have failed. The danger is only on hot summer days, when the gases inside a Koffing expand, the pokemon explodes, and vast amounts of toxic gas are released into the atmosphere. During a recent heat wave, Celadon's air was fatal to breathe without protection such as gas masks for a whole month.
At the moment, Koffing are becoming more dangerous by the year. For whatever reason (Moltres, Ho-Oh, Entei, or Groudon's power growing, or perhaps merely a greenhouse effect) the pokemon world is undeniably warming, and more Koffing are exploding in industrial centers every year. They are created faster than they are captured or explode, and it is feared that as planetary temperatures continue to warm, the cities of the pokemon world will become deathtraps.