Iodopropynyl butylcarbamate (IPBC) is often used as a preservative in lotions and personal care products due to its effectiveness at preventing fungal growth in topical products.

From the years 1996 to 2001, the Food and Drug Administration reported IPBC to be the preservative with the fastest growth in use in cosmetics.

The maximum level for safe use in leave-on and rinse-off products has been set at 0.1 percent by international authorities.
Furthermore, an irritant label must be placed on all products containing concentrations greater than 0.01%, although cosmetic products are allowed to have 10 times this concentration without labeling.

Observing for IPBC allergy is important since the frequency of contact allergy may rise with the increasing availability of IPBC-containing cosmetic products. IPBC can lead to sensitization after extensive and prolonged exposure, but only a few cases of ACD have been reported from its use in cosmetics even with its extensive use.

Sources
J Clin Aesthet Dermatol. 2008 Nov; 1(4): 38–44.
Moisturizer Allergy Diagnosis and Management
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3016930/