That could be solved with an adequate partitiion installed (partition seperates the cab from the cargo). Many places require a work van to have a partition to prevent tools from entering the cab area when driving. The partition could be made to be comepletely air tight blocking fumes from the back. A sparkless ventilation system could even be installed if required.

This all may sound like an expensive upfront cost but it sounds worth it to keep expensive welders and such dry from rain and clean from road dirt.