Most of Cologne’s metrolpolitan area is a low emission zone, into which only vehicles sporting one of the stickers for emission group 4 (green sticker) are allowed to enter since July 2014.
The Koelnmesse exhibition center is, however, accessible for all types of vehicles, even if they do not bear one of the afore mentioned stickers. You will find directions on how to reach the site without a sticker at map low emission zone map (.pdf) including highway connections.
