Do you know the differences?
I assumed the forensic suits were similar, and there are just different levels of protection. Sometimes, as in the Millard farm inspection, I understand they brought in students to help comb the area (maybe criminology students), and the outfits were more or less to help distinguish who is who.. In the Millard farm photos you have both the guys with full face mask (maybe the old pig farm really stank?) and you also have those in lab coats and low level dust particle masks on. I'd think just being allergic to environmental allergies someone may dress in that gear. I wear a breathing mask when I cut my grass.
When my brother goes on certain projects he wears a safety vest even though they'll be in a field and it is more or less so the public knows there is work going on and they aren't randoms.
This article is kind of interesting, and it points out one case they wore them simply because the agricultural land they were checking had been fertilized
http://www.crime-scene-investigator.net/hazards.html