I've a dust mask but I wonder if it worth wearing it when working on
pine wood. I do wear it when working a piece oak or beech.
First off, let's be clear: a disposable paper dust mask may be slightly better than nothing, but even a fancy $5 disposable P100 dust mask with a breathing valve will not perform as well as a properly-fitted respirator with replaceable P100 filters. If you have a beard, you should consider a positive-pressure respirator since a normal respirator may not be able to make a good seal with your skin.
Some woods are more toxic than others, and some people are more sensitive than others to certain types of wood. The Wood Database contains toxicity information for many types of wood, including possible symptoms.
Would you consider cutting some wood species wood without mask safe ?
Do you use mask with every wood species ? how long dangerous dust stay
in the air ?
I consider cutting any wood without a respirator unsafe, regardless of species. The length of time dust remains in the air will vary, but I've seen at least one magazine article recommend continuing to wear a respirator for at least 30 minutes after you stop producing sawdust in that room, assuming the air is not disturbed by a fan or air handling system.
Also, is mask still necessary when using a central dust collector ?
Would you rather have your lungs or a respirator filter the particles out of the air?
If you want to err on the side of safety, you need a properly-fitted respirator with P100 filter cartridges. Few, if any, dust collectors are capable of capturing 100% of the dust at every tool (it also depends on the tool's design), and very few dust collectors provide filtration adequate for removing sub-micron particles. Most inexpensive consumer-grade dust collectors only filter down to 1, 5, or even 30 microns. A dust collector outfitted with this type of filter will simply recirculate the sub-micron particles through the air, allowing them to increase in concentration as you continue to work.
If exhausting your dust collector to the outdoors is a practical option, you may want to consider that rather than spending a lot of money on HEPA-rated dust collector filters. The obvious drawback is that you will also exhaust all of your warm air in the winter and all your cool air in the summer. Also be certain to include an air inlet if exhausting outside. Otherwise, your dust collector won't work as effectively; and worse, negative pressure inside your shop can cause the combustion gases from a water heater or furnace sharing the same space to be pulled back into the space--for example, if your shop is inside your basement.