It seems that almost all claims are rejected for one reason or another. This is standard for Veterans Affairs Canada so don't be discouraged.  You will need to file an appeal. Fortunately, there is a department that will help you with the process the Bureau of Pension Advocates (BPA)

Bureau of Pension Advocates

If you have made an application for disability benefits and received a decision from Veterans Affairs Canada and if your application was rejected or you received less than you expected, you can review or appeal that decision, and the Bureau of Pensions Advocates can help. Click here for their website: Bureau of Pensions Advocates

Although they are very helpful you will have to do most of the leg work yourself. For example you will need a statement from someone who can verify that you used Carbon Tetrachloride while in the service. They will also ask you to get a letter from your doctor stating that your cancer is related to your use of Carbon Tetrachloride. This is virtually impossible since no doctor can make such a statement. Only research scientist who have researched Carbon Tetrachloride as a known carcinogen could possibly make such a statement. This is why documents making this connect are important. You can find some of them on this website.