Human Skin Permeation of Ethoxy- and Propoxypropanol Commonly Found in Water-Based Products

Abstract

Some propylene glycol ethers (PGEs) have been associated with reproductive toxicity. Ethoxypropanol (PGEE) and propoxypropanol (PGPE) are two common PGEs found in many commercial products. Although skin exposure is frequent when handling such products, no studies have investigated their skin absorption. Neat or aqueous concentrations of PGEs were applied with different concentrations on previously frozen human skin according to OECD guidelines. We also explored the use of frozen skin for skin irritation screening. Our results show that both PGEs readily permeate human skin (permeation coefficients: Kp_PGEE = 0.0005-0.002 cm/h; Kp_PGPE = 0.0002-0.002 cm/h; rates: J_PGEE = 447.5-1075.2 µg/cm²/h; J_PGPE = 193.9-826.1 µg/cm²/h; and time lag: 2-5 h). The permeability rate was four times greater for PGPE diluted in water compared to neat, and double for PGEE. Increasing the water content increased PGEE skin permeation but had no effect on PGPE. Cleaning products contain 1-5% PGEs, and water-based paints 10-50%, thus increasing the potential for skin uptake in consumers. Our skin irritation results were inconsistent, so we conclude that skin irritation cannot be assessed with previously frozen human skin. Future studies should assess the irritation using fresh skin and investigate the risk of health effects from PGEs exposures.

Keywords: biomonitoring; exposome; organic solvents; risk assessment; skin absorption.

Figure 1

Mean cumulative amount in the receptor fluid (mg/cm²) (y-axis) of neat or aqueous PGEE and PGPE over time (x-axis) in a flow-through diffusion system (n = 3, from three different human donors) with in vitro skin permeation parameters (J, Kp and Tlag) for PGEE and PGPE estimated from their permeability curves. Skin permeation of studied glycol ethers during the first 3 h of exposure (A) or 24 h of exposure (B). The last column of the table represents the percentage of the cumulative dose quantified in receptor fluid samples after 24 h.

Figure 2

Skin permeation experiment of PGEE and PGPE through static diffusion cell system (n = 3). Mean cumulative amount after 24 h of exposure (µg/cm², y-axis) according to different concentration of propylene glycol ethers (x-axis). PGEE and PGPE were undiluted or diluted in 25%, 50%, or 75% of water and applied on previously frozen human skin (n = 3 cells for each condition).