What kind of wood chippings are they? Pine and cedar are both potentially toxic for rats. These are soft woods that contain aromatic chemicals called "phenols". These chemicals have been shown to cause respiratory damage as well as liver disease. A safer alternative to pine and cedar is the hard wood called aspen. Aspen, being a hard wood, contains no harmful phenols.
Are these actually chips or shavings? Aspen shavings are ok for rats, but chips may be more uncomfortable. I personally can't use aspen, as I am allergic to it. Before I find it isn't very absorbent either. There are numerous alternatives though:
Paper-based: These beddings are made out of paper. Two popular brands are "Carefresh" and "Yesterday News", though others are available. These are more absorbent of odor and moisture, and softer for more comfortable nesting
Alfalfa pellets or aspen pellets: I have found pellets are extremely absorbent, but they also break down as they absorb. These can be extremely heavy, but a little goes a long way. If you have allergies to aspen or alfalfa, these won't be very good.
Soft Sorb: I have never personally used this, but have only heard good things about it. Apparently it is supposed to be very absorbent and very comfortable for rats to nest in.
Corn cob: Not very absorbent, this bedding can be used. I have read reports of this bedding getting moldy, though have never personally experienced that.
Many people also use fleece as bedding. This does need to be changed daily, but can be thrown in the wash and reused.
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