The Biology of Polycyclic Aromatic Hydrocarbons (PAHs)
Airborne urban pollution consists of Particulate Matter (PM2.5)—microscopic particles measuring less than 2.5 microns in diameter (by comparison, a human pore is approximately 50 microns wide). These particles are coated in lipophilic Polycyclic Aromatic Hydrocarbons (PAHs) from vehicle exhaust.
Because PAHs are oil-soluble, they pass straight through the skin's lipid barrier. Once inside, they bind to the Aryl Hydrocarbon Receptor (AhR) inside keratinocytes, triggering a massive release of inflammatory cytokines (IL-1alpha, TNF-alpha) and activating Matrix Metalloproteinases (MMPs) that physically chew up dermal collagen and elastin fibers.
The Post-Commute Cleansing Protocol
Simply rinsing with water does not remove lipophilic soot particles that have bonded to facial oils during your subway or traffic commute.
Incorporate an oil-based cleansing balm followed by a gentle amino acid foaming cleanser every evening. The oil dissolves the lipophilic soot, while the gentle surfactant flushes the heavy metal particles down the drain.