As mentioned, mask on. If you want to be extra cautious, have someone water with a watering can while you saw to completely avoid the dust.
There are low levels of asbestos in fiber cement boards, so one shouldn't exaggerate the dangers of this (for those of us who don't work with it daily) - BUT it costs very little to put on a proper face mask - so why not?
EDIT: By the way, can I suggest that you replace these pipes while you're at it? Seems like a great opportunity that's presented itself.