A good point! I forgot about that.

The best way to avoid that is to give a lab its own fresh-air intake at least of the size of the exhaust. However that could mean getting too cold air in. Aside of an airheater or heat-recuperator, the other way would be to get the "fresh" air in by many "leaks" from the rest of the house. Preheated so to say. But if this doesn't work correctly a drain on that heater system might be produced, and exhaust from that may come in.