[MargaretR]

My windows have 'trickle vents' - some can be shut and some can't.
The ones that can't, I blocked with polystyrene foam.
I choose the time when windows are open for an air change -midday when the temp is at its best.
It is important to ventilate rooms that have gas appliances, because they use up oxygen in the burning.

If I was in your shoes I would block that vent - but ensure your own safety by changing the air in the room for half an hour every day - and get a carbon monoxide detector