As a guy who has lost a fair amount of brain cells to paint and varnish vapors I know better than most that many projects aren't properly ventilated. (Yes I called them paint and varnish fumes in the title because that's what folks search for. Fumes are heated solid particles - think burning metal. Vapors are gaseous. I haven't lost all my brain cells.) That being said, there are some jobs that even I know have to have a fresh air source.
Respirators are great, but in an environment with heavy vapors from paint or varnish, they can quickly be overwhelmed. Respirator cartridges are like sponges. Sponges can't hold all the water in the sink. Likewise, cartridges are great at whisking away some of the organic vapor released when painting, but they can't overcome heavy concentrations. If your job doesn't have proper air flow, you can quickly find yourself breathing in air that is highly toxic.
The solution is ventilation. And it can be a pretty straight-forward operation. The Allegro 8 in. blower with built in ducting and canister 9533-25 can be quickly deployed, has heavy-duty, reinforced ducting, and delivers a massive amount of fresh air where you need it or to push bad air out of the area. On the sites I've worked in, an air conditioning filter was often employed over the air intake point (typically on the far side of the space from the blower). The contaminated air was pushed out by the blower, and the filter served to keep the incoming air free from particles that could ruin your paint or varnish work.
The 8 inch blower moves just over 830 cubic feet per minute (CFM) when deployed in a straight line (without any bends in the ducting). If a space with significantly greater volume needs to be cleared, Allegro also makes a 12 inch blower with ducting and canister. The 12 inch model moves over 1800 CFM in the same configuration.
If you are painting in an enclosed space, you need to ventilate. Don't fool yourself into thinking it's a quick job and you'll be out of there soon. We all know it never happens as quickly as we'd like it to. And damage to lungs and brain cells is serious and lasting business.