First check if existing ventilation works. In (old) apartments, leaky doors and windows works as intake, and air go out from toilets and kitchen ventilation. Ho

First check if existing ventilation works. In (old) apartments, leaky doors and windows works as intake, and air go out from toilets and kitchen ventilation. Hold a toilet paper tissue at the outtake in the toilet or kitchen, the air flow should be big enough so that the paper get stuck. If it doesn't - try opening the door or windows. If it helped to open the door, or window, you can install window ventilation, which is basically a hole with an adjustable opening. If opening the window doesn't help you need to move the air mechanically by installing a fan in the kitchen outtake and/or the toilet. A fan might solve the problem for you, but might push the air (and smell) into someone else's apartment ... so it's best to work with the rest of the house, and do a whole house solution, sometimes it's as easy as cleaning the ventilation channel, as birds love buildings nests in them because warm air make it cosy.