Can my employer force me to work in poison oak?

I work for a city as an outdoor landscaper. I was recently assigned to work on hiking trails that are littered with poison oak. Despite doing my best to cover up, rinse with soap and water afterwards, and to avoid it I have become highly susceptible and allergic to the poisonous plant. Out of a department of 30 landscapers I am literally the only one assigned to work in an area where poison oak is present. Since I am so highly sensitive is my employer being negligent for not assigning me to work in an area safer for my allergy, while assigning another worker with a high resistance to the allergy to work the trails? I am also being told that I cannot go to see occupational health on my employees time or money even though I came in contact with the allergen during work. I have had 10 separate reactions to exposure from the plant with some cases 2/3s of my body being covered in unbearably itchy rashes.