The Material Safety Data Sheet (MSDS) for RI Brick and the warning on the packing boxes should discredit this idea. There are very direct comments on RI Brick packing boxes such as "This product contains a substance which has been identified by the International Agency for Research on Cancer (IARC) as a known carcinogen to humans", the product being silica and/or cristoballite, known and accepted as causing cancer.
RI Brick is a Category 1 hazardous product which can cause cancer. Ceramic fibre is classed as Category 2 (still disputed) which is defined as a product believed to cause cancer. Of course, when studied in depth, all these warnings and comments in the MSDS apply to the INSTALLERS of the products, NOT the final users. When you are loading or unloading a kiln, you are not going to drill holes or cut the bricks in your kiln, no more than you will tear chunks out of your fibre lined kiln. A bit of common sense would help here as well. I believe that this is similar to the asbestos scare, where people started to believe that if they as much as lived in a "fibro" house, they would soon die an excruciating death from lung disease.
Maintenance is always the key to safety in any field. If you decide to abandon the maintenance schedule on your motor car, you shouldn't be surprised if one day it fails to stop, runs off the road and tries to kill you. A pottery kiln deserves the same consideration. CONCLUSION : RI BRICK and CERAMIC FIBRE are BOTH as SAFE AS YOU MAKE THEM, or as DANGEROUS AS YOU MAKE THEM. FINALLY: I never cease to be amazed by customers who tell me that ceramic fibre is dangerous, then proceed to produce a great cloud of clay dust while smoking a cigarette. It's all relative!