Rain coat alternative

Does anyone know of a chemical-free alternative to a water-resistant rain coat? I have chemical allergies and develop skin rashes from just about everything synthetic and man-made. If you're like me, you know that items that are safe and common place for most people, like rain coats, are forbidden for us. I'm trying to find a rain coat that doesn't have chemicals, especially near the neck. Thoughts or suggestions, anyone? Thank you and take care!