To play as the Scientist, and thus unlocking this achievement, you'll need to pollute five different air filtration systems.
This is cumulative over multiple playthroughs, but can be done in a single run. Firstly, you'll need to find a syringe which can be found in trash cans, as drops from killing Scientists, or bought from a Drug Dealer.
Syringes come in different flavours and colours. They have positive and negative effects, but won't hinder your progress towards this achievement, just use any and all that you manage to find.
One you have a syringe, you'll need to find an air filtration system which can be found attached to the outside of buildings. Once you've located one, simply insert a syringe into it and watch the panic ensue inside the building.