There are forty Biohazards to burn throughout Los Perdidos, ten in each zone. These become available after Nelson asks you to wait while he works and is also the optimal time to collect them in conjunction with all other collectibles. They are marked on your map by a solid yellow circle and a green flare will indicate which pile of bodies you must burn. To burn a pile of bodies you must either shoot or melee them with a fire weapon. The DLC is filled with a new gun called the 'Slagshot' and these are perfect for this achievement.