Is it safe to seal up all the air vents?

Is it safe to seal up all the air vents?

Don’t undertake draughtproofing if you have an unflued gas heater or any open flued heater as, in a tightly sealed home, the gases from such appliances can present a carbon monoxide poisoning risk. Refer

Air needs to flow in and out of your house, so it stays fresh, dry and healthy. Make sure you don’t block or seal any intentional ventilation, such as the following:

Extractor fans – take out damp air quickly in rooms where lots of moisture is produced (kitchens, bathrooms and utility rooms).
Under-floor grilles or airbricks – help keep wooden beams and floors dry.
Trickle vents – modern windows often have small vents above them to let fresh air trickle in.

Avoid or replace open-vented downlights that penetrate ceiling insulation  - Lighting that emits high levels of heat (e.g. halogen) requires clear space around it so it does not come in contact with the roof structure or insulation (see Lighting). Proprietary heat-resistant isolation boxes are available but often cost as much as CFL or LED replacement lamps and can be difficult to fit. Flexible, sealed downlight covers are available. Otherwise an electrician can replace your downlights with IC-4 rated LED downlights which allow insulation contact.

Electrical outlets -   a common and particularly problematic source of leaks. Sealing around switches and power points can help but this makes them difficult to remove for maintenance. Draught-proof building wraps should be breathable to avoid dew-point formation. Where this is unachievable, a qualified electrician may be able to fit self-adhesive seals to the back of plates or seal problematic outlets with spray foam injected into the air space.
Do not use permanently ventilated skylights - These allow hot air to escape and cold air to enter. Use a diffuser at ceiling level and a separate ducted exhaust fan with self-closing baffles.

Seal off permanent air vents - Use windows and doors or heat recovery systems for ventilation as required by climate.
Insulate manholes or roof space access hatches - (which are often poorly sealed and uninsulated) with rigid insulation fixed to the hatch. Fit air seals to all sides.