When your range hood is exhausting, if it doesn’t have adequate makeup air to replace the air that’s getting exhausted, it won’t work as it should. Think of drinking out of a straw that has fruit remnants at the bottom of it… it’s not going to work that well. Check out the video to see how much a range hood can depressurize your house and different methods to help with equalizing this pressure via passive or mechanical makeup air.
The building code requires makeup air if a range hood is in excess of 400 CFM (IRC M1503.6), but it turns out makeup air may be required for any range hood if your house is airtight. Makeup air allows the range hood to operate efficiently and also ensures proper indoor air quality so that any open combustion appliances aren’t backdrafting into the house.