generally sawdust is as the title suggests, ie contains dust, you cant get dust-free sawdust.

its beneficial to rats to have dust free bedding/substrate as they are quite delicate, much like humans, hence why threy test so much on rats..
this is a good site for anything rat related www.fancyrats.co.uk

`litter for the base of the cage/litter trays`
There are several options when deciding which litter to use in the base of your rats’ cage, and what you use will probably be determined not only by what you prefer but also by what you have available to you in your area. Any litter should be dust extracted and odour free.

Unsuitable cage litters
Wood Shavings – pine, spruce and cedar shavings are one of the most commonly seen litters but in fact, there is evidence that suggests they may be harmful to the health of rats and other small animals. As well as generally being quite dusty, these softwood shavings give off phenols and acids that are toxic and could lead to irritation of the respiratory tract and skin, and liver disease. It is possible that in some rats this could reduce life expectancy. There are many other more suitable alternatives.

Sawdust – all of the above applies to sawdust with the added problem of it’s fine grain which renders it very dusty.

Pine cat litter – wood pellet cat litters form sawdust when wet, and have all the issues of shavings/sawdust.