The CSA Standard W117.2 states that contact lenses should not be worn by welders and welding personnel.  Contact lenses do not provide protection from ultraviolet radiation and flying objects. All workers in proximity to welding procedures must wear appropriate eye protection according to the circumstances. The OSH Answers document Contact Lenses at Work discusses how dust particles or chemicals can irritate the eyes.

Note that in Canada, Prince Edward Island's Occupational Health and Safety Act Regulations (E.C. 180/87) Section 45.11 specifically bans wearing contact lenses while welding.