This type of chemicals were used, because they penetrated through gas masks and made the soldears to tear their gas masks off. In combination of more toxic war gases, like the phosgene, the irritants made the defence against gases harder. It is easy to understand that SS preferred not to use original Zyklon B with irritants and warning agents. I'm thankfull for your kind help. Here in Finland we don't have very detailed knowledge about the Holocaust.
More detailed books are hard to find.
It is not a part of our history because the Finns were not involved in it. Luckily this doesn't mean ignorance, and our schools teach the subject in a objective way. There have been no serious attempts of denying the Holocaust here. I wish you good luck. I am contacting you to see if any progress has been made, or if it is worth posting the enquiry in full again.