I'm considering buying Manfrotto 055CXPRO3 carbon-fiber tripod legs with a Manfrotto 498RC2 ball head, but I've read that carbon fiber can break at low temperatures. (Note that the 055CXPRO3 uses pull-wound carbon fiber tubes.)
Carbon fiber can take quite a hell of a beating, both in terms of environment (water, sand, snow) and temperature. I've heard a lot of people discussing or complaining about how carbon fiber is susceptible to extreme cold, however I think most of it is hearsay and speculation.
There are only a couple times when I've read something regarding carbon fiber being fragile at cold temperatures...but the use of the word "cold" was always lacking. You would need to spend a night out on the frozen ice pack of the most northern reaches of Alaska during the heart of winter (which is actually what all of the verifiable accounts of broken carbon fiber 'pod legs I've read described...usually in relation to photographing the auroras) to actually experience cold enough temperatures for it to actually pose a serious problem. Most carbon fiber tripods these days are multi-layered, weaved or braided, and resin-reinforced. In the average case for cold, you may crack a layer if you slammed a leg with enough force, but it would take a pretty extreme freeze, well, well below zero, to run the risk of actually cracking a leg all the way through or shattering one completely.
As for precautions, if you are using your tripod at -20 or below, you need to make sure you don't slam it or strike it with anything hard. Dropping your pack or bag on hard snow pack or ice with the pod strapped to the outside is a sure way to crack or shatter it at extreme temps. You can crack a leg by pulling it too hard when setting it up, so you need to be careful with that as well. Section releases can ice up, and wrenching them too hard at cold temperatures may cause cracking, so its best to ease into it. Better yet, extend one or two of the sections before you get out into the extreme cold, and only extend the last sections if you absolutely have to (most of the articles I've read on aurora photography seem to indicate that you'll probably be sitting in a chair anyway, so full extension is often not required in the first place.) Once set up, you'll want to make sure you don't slam any of the legs with anything hard...such as the camera, any other gear, a flashlight, etc.
If for whatever reason you DID shatter a leg, I don't know of any specific health hazards. I've always heard carbon fiber was a pretty stable, safe, environmentally friendly product, so I wouldn't worry about that. Its unlikely it would break cleanly, especially if you have a layered product like a tripod leg. Its likely to leave a rough edge, so I would be aware of that and make sure you don't scrape your stab yourself with any broken ends.
It might be worth considering that modern aircrafts (like A380) have a large amount of composite materials, including carbon fibre. To cite Wikipedia "The A380 is the first commercial airliner to have a central wing box made of carbon fibre reinforced plastic". Flying at almost 40 000 feet and experiencing temperature as low as -40 every day is proof to me that carbon fibres can take the stress.
The question is if tripod manufactures are making strong enough composite materials or not in their products, not if carbon fibres are strong enough in (very) cold weather.
As for health hazards, you should treat carbon fibres as most other composite materials. It's not a good idea to breath in the "dust" from the resins and such, but this isn't really a problem outdoors. People that work with composites have to take precautions with the fumes and dust, but it's not really a problem during a "small" failure. The splinters from the fracture area might be very small and sharp, and could penetrate your skin. I haven't heard that this is dangerous, but it will itch, just like fibreglass composites.
I have a similar Manfrotto CF tripod, I have not had issues down to about 10 below so far.
I am not sure what the failure mode looks like.
The precautions to take would be to avoid hitting the tripod legs on hard surfaces in very cold conditions.
CF legs are much nicer than the Aluminum legs I was replacing for cold weather use, as they do not chill your hands nearly as much - you should probably still use gloves, but you can often get away even in very cold conditions with just liner gloves for a while. So there is a significant and more likely upside to buying CF legs for use in cold weather conditions.
I have a RRS TVC-33 and love it, but i do have issues with the leg locks in cold weather. The problem i have is that when your out in cold weather and particularly when there is a light snow my tripod leg locks will freeze and i can no longer move them and am stuck without being able to collapse the leg sections. I also have my older manfrotto 055xprob aluminum tripod and have to revert to it many times in cold weather.
I can't speak to the above issue because I don't know the tripod however carbon fiber is not as simple as you'd want it to be. The reason there is no 'true' answer to your question is because there are different bonding agents used to make different versions of carbon fiber. The carbon fiber epoxy used in a Boeing 777 will be able to take temperatures well below anything you'll experience on earth. It also depends on the thickness of the carbon fiber.
You'll need to test it i.e. when you see a broken one in the garbage, grab it put it in the freezer then smack it with a hammer and see what happens.
