When a person has a blockage in their sewage system they often try to fix it themselves by adding an acid or a base such as Drano in an attempt to dissolve or dislodge the problem. These chemicals can get into the plumbers eyes  when the sewage is splashed during the repair. The plumbers skin during the repair does come into contact with the sewage water. The owner of the toilet might not report to the plumber they have already tried Drano a highly caustic base .
Most states and localities require plumbers to be licensed. Although licensing requirements vary, most states and localities require workers to have 2 to 5 years of experience and to pass an exam that shows their knowledge of the trade and of local plumbing codes before they are permitted to work independently. In addition, most employers require plumbers to have a driver’s license.