Negative Pressure

Just wondering if negative pressure is an issue when growing indoors? My room is a storage room in my garage. Its 1mx3m and about 2.5m high. Im going to purchase maybe a 6" or 8" fan to extract the air efficiently but im not going to have any fresh air coming in except for a few gaps in the brickwork in the storage room I can see one gap that is at least 2"x8" so its not that big along with a few other small cracks and crevasses so it definatly will have fresh air coming into the room but not sure if its going to even it out. Would negative pressure be an issue if only a small amount?

Should be fine with a 6" might need to bust out another brick for an 8"... give it a shot.

intake should be about twice the size as exhaust. You should probably widen a gap in the bricks.

Id say with the main gap and all the other small cracks and spaces it should be sufficient enough. Just wanted to know if there are downsides to negative pressure. I heard a positive is it keeps the smell in

No, negative pressure keeps the smell in; that's what you want. With positive pressure you would be forcing hot skunky air out of those cracks all over the place like inside a balloon.

Always pull the air, never push. Pulling creates a vacuum and the best way to test if you have achieved negative pressure is to put a lighter next to your intake and if the flame pulls towards the exhaust you should be good.

I have a 5' x 5' closet in my room, i use 4" inline, pulling through a filter and cooling my 400w. i sealed up the door so its as air tight as possible and i have good negative pressure. My passive intake is the 1/8 inch gap under the closet. my temps stay between 73-78F. Ur gaps should be good to pull fresh air.