In this post, Heidi asks...My question is this.They tell us we don't need preservatives in soaps. Do you believe this is true? And if it is, why do we need a preservative in a solid shampoo bar (syndet)?And how about something without water like a body lotion bar?
Great question! Let's take a look at bacterial contamination in soap first.
Washing hands with contaminated soap bar unlikely to transfer bacteria. This is a study from 1998 and seems to be one of the most quoted ones. In essence, the title says it all.
Microbial contamination of "in use" bar soap in dental clinics. This one showed that soaps in a dental clinic can be contaminated by microbes, but it doesn't note if those microbes are transferred to other users. This sentence caught my eye: "The Center for Disease Control -1989 chapter 8, suggests that bar soaps should not be used for hand washing." I'm hoping this is just in medical settings!
Bacteriological studies related to hand washing. The conclusions in this one are interesting (last page). In quick summary, bar soaps do not support the growth of bacteria and said bacteria couldn't be passed on from one person to another. Bar soaps are anti-bacterial by their physical and chemical nature, but even if they were contaminated, this wouldn't be passed on to someone else. But this was published in 1963, so who knows if it's still relevant information?
I'm actually quite surprised there haven't been any more large, recent studies on this topic (almost everything else I found had tiny groups - say 6 people - or was about anti-bacterial soaps). Having said that, the general consensus appears to be that soap bars may harbour some contamination, but it's not being transferred to the user. Suggestions for keeping soap from obtaining more contamination are to keep it in a well drained container and to keep it dry.
As for anhydrous products like a lotion bar, every day microbes don't want to grow in oil so it's fine to leave the preservatives out of products that don't contain water and won't be exposed to water.