If it's the asbestos cement board provided you don't saw/drill/snap it then it's not a danger. Even if you sealed it, you still shouldn't do any of those things. Just leave it be, and if you need a new roof, get it removed carefully. If on the other hand it's some kind of insulation, be very careful and get a specialist in ASAP as that kind does need sealing in place or removing by a specialist.
Dh sealed ours, he knows not to saw of remove any of it and as it was in great shape and not leaking he sealed it with some sort of black looking stuff he bought online after a lot of research. Unfortunately he;s away till Wednesday so I can't what it was called.
Sorry I didn't get back sooner he finally got here and I went to bed, then working long days. He said he used Black Jack All weather roof coating, and it was cheapest with free delivery from Toolstation if there isn't one near you for pick up. Roof has to be all dry before using it. black jack