Ceiling vents for exhaust any time are a real bad call, whereas it should be sealing ceiling vents asap for all seasons. Some roof ventilators depend on ceiling vents for inlet air, this can contribute to serious health hazards.
An open ceiling vent for exhaust is also an open ceiling vent for entry – entry of all the negatives of the roofspace to enter the enclosed space (the home). The main offender microscopic fibres and larger particles from fiberglass and other types of ceiling insulation whilst these other types of ceiling bat insulation including fiberglass when disturbed release fine particles. These particles could further exacerbate or cause lung irritation or any type of respiratory infection. Although other than asbestos these particles are not believed to be carcinogenic yet, only time will tell.
There are products to cover ceiling fan outlets however these are also collectors of particulate matter and when open allow entry of airborne particulate matter to enter the home.
As well as the possibility of particulate matter from the insulation, the insulation is a collector for dusts, moisture, moulds, termites etc etc.
CEILING VENTS FOR EXHAUSTS IN SUMMER OR ANY SEASON ARE A DEFINITE CONTRADICTION TO A HEALTHY HOME
If you have an older tastic bathroom fan/light combo you only need to look around the lights to see how many fibres have collected to see what you may have been breathing, we can’t see minute particles that are airborne but we can certainly breathe them in.
So seal your ceiling, duct all outlets to the outside atmosphere – ceiling vents, ceiling fans – bathroom, kitchens etc and ventilate your roofspace to get rid of all the negatives of the roospace with an effective roof ventilation system with inlets and outlets in the roof covering.