asking to be relieved from field testing at work - help! (long)

Ms H13

OK ladies, I need your help here.  I work in an R&D dept for a pest company.  I do things such as identify insects and test the efficacy of pesticides.  I am currently on a project to develop a new treatment protocol for our field crew and have been expected to do field trials once a week since June.

These field trials involve arriving at the first location early in the morning (5:00am), inspecting traps set out the previous week and applying pesticides.  When I found out I was pregnant, I set up some meetings with my HR rep to ask about my company's policy for pregnant women doing field work.  The HR rep said she would look into it and about a week later I got an email from her saying there should be no concerns as long as I wear my protective equipment (gloves, goggles, ect).  To be honest, I was expecting her to say that the company policy was for pregnant women to have their roles adapted such that their exposure to chemicals (especially pesticides) was minimized.

 I have my update with my boss next week and plan to tell him I am pregnant.  I also plan to tell him that I am uncomfortable continuing with the field trials.  Any advice for convincing arguments - also feel free to chime in if you think it is unreasonable to request a role change here.  TIA for any suggestions!