Chemistry of skin lipids

Human body is covered with skin surface lipids. The majority of skin lipids are sebaceous lipids which are secreted by sebaceous glands.  Epidermis also produce a tiny fraction of total extractable surface lipid. On skin areas rich in sebaceous glands such as forehead,  epidermal  lipids are 5-10 µg/cm3 compared with 150-300 µg of sebaceous lipids.

Chemicals that constitute skin surface lipids include non-polar lipids, mainly triglycerides, wax esters, squalene, fatty acids and smaller amounts of cholesterol, cholesterol esters and diglycerides

Carbon-carbon double bonds involve in many of these constituents.  Such chemicals are readily to react with ozone. On a molar basis, squalene is responsible for roughly 50% of the unsaturated carbon bonds in skin surface lipids. Hence, squalene is the most important individual constituent in terms of ozone consumption.

The fractional contribution and  levels of skin lipids change with human ages. For example, the amounts of omega-7 unsaturated fatty acids

 have been found to increase with increasing age

Reactions between ozone and skin lipids generally follow the Criegee mechanism

(Youtube education material on Criegee mechanism of ozonolysis)

The ozone-skin lipids reaction products cover a range of volatility. These more volatile products are found primarily in the gas-phase while the less volatile primary and secondary products are found primarily in the condensed phase (e.g., skin, hair, clothing surfaces,  airborne particles,surface film).

Based on the Criegee mechanism, the ozone/squalene reaction is anticipated to produce aldehydes and organic acids with 27, 22, or 17 carbon atoms and five, four, or three double bonds, respectively and hydrogen peroxide (H2O2), organic peroxides (ROOR) , and short-lived highly reactive products including hydroxyl (OH.), hydroperoxyl (HOO.), and alkyl peroxyl radicals (ROO.). These radicals will react with squalene and primary and secondary products producing additional carbonyls, dicarbonyls, and hydroxycarbonyls.  Given the number of double bonds in these less volatile products, they themselves readily react with ozone to generate still more products.

The reaction could generate products with high hydrophilicity, which increases their transdermal penetration and redox activity. Such products from ozonolysis could pose potential risk  to human health.

image source: http://www.cdc.gov/niosh/topics/skin/

Some of the primary and secondary ozone/lipids reaction products can condense on existing particles or nucleate to form new particles or secondary organic aerosols (SOA) with size ~10-1000 nm. Based on chamber experiments, the size of SOA formed depends on ozone concentrations with larger size at higher O3 concentrations. 

The reaction of O3 with lipids at skin surface involves the deposition of ozone to skin surface and the chemical reaction.  Various studies suggest the actual reaction is very fast and the overall rate limit step is the deposition of ozone to the surface. 


References and further information: 

• Weschler CJ, Roles of the human occupant in indoor chemistry. Indoor Air. 2015 http://dx.doi.org/10.1111/ina.12185
• Apostolos Pappas. Epidermal surface lipids.2009 1(2): 72–76
  http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2835894/pdf/de0102_0072.pdf