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J. Cosmet. Sci., 75.5, 378–407 (September/October 2024)
*Address all correspondence to K. P. Anathapadmanabhan, ananthky@ucmail.uc.edu.
Interactions of Cosmetic Ingredients With Human Stratum
Corneum
K. P. ANANTHAPADMANABHAN
James L Winkle College of Pharmacy, University of Cincinnati, Cincinnati, Ohio, USA
Accepted for publication December 02, 2024.
Synopsis
The human stratum corneum (SC) is a physical barrier that prevents excessive water loss from the body and
prevents the penetration of external pollutants and chemicals into the body. It is also a microbial barrier that
prevents germs and microbes from entering the body. Skincare is all about protecting the SC barrier and
ensuring its water holding and viscoelastic properties. The SC is routinely exposed to a wide range of chemicals
including cleansing agents, moisturizers, and other advanced skincare products. Ingredients and products can
interact with the SC barrier—some leading to weakening the barrier, and others to strengthening it. In this
article, interactions of commonly used chemical ingredients with SC and their impact on SC barrier properties
are reviewed, and approaches to identifying barrier-friendly ingredients are discussed.
INTRODUCTION
The stratum corneum (SC) is the main barrier that prevents water loss from the body and
prevents the penetration of chemicals, pollutants, and germs into the body. Maintaining a
healthy SC is important not only for visibly good-looking skin, but also for ensuring overall
health and hygiene. The SC, however, is vulnerable to damage from changes in environmental
conditions, such as temperature and humidity, and from the chemicals and pollutants in the
atmosphere. Some chemicals, such as surfactants in cosmetic and personal care products,
can also lead to alterations in the properties of the SC, resulting in a weakening of the
barrier. A combination of factors—for example, a harsh cleanser with low temperature and/
or low humidity—can exacerbate the SC condition, resulting in a poor SC barrier. The
consequence of a poor skin barrier is the penetration of irritants into deeper layers, resulting
in a biochemical response from the body that often manifests as dry, itchy, and inflamed skin.
Ingredients found in moisturizers such as occlusives, humectants, and lipids can enhance
barrier properties. Selection of cosmetic ingredients based on an understanding of their
interactions with the SC barrier will help minimize and will even eliminate the harmful
effects of cosmetic ingredients on skin. In this review, the impact of some of the common
classes of chemicals used in cosmetic and personal care products and the ways to minimize
their damaging effect while maximizing their beneficial effects on the SC are discussed.

379 The Human Stratum Corneum
BACKGROUND
The human SC, the uppermost layer of the skin, consists of about 70% proteins, 15% lipids,
and 15% water. The simplistic view of SC as a “brick and mortar” structure consisting of
corneocyte “bricks” embedded in a continuous lipid matrix made of ceramides, cholesterol,
and fatty acids has been refined over the past three decades, due to more information
on the composition and organization of lipids and proteins in the SC.1–5 The SC, which
was thought to be a dead tissue in the 70s, is now considered to be a hot bed of enzyme
activity.3 The role of the skin microbiome in maintaining a healthy barrier is also receiving
increased attention over the past decade.6–8 Recent advances in our spectroscopic and
imaging techniques including in-vivo techniques9–13 have made it possible to characterize
the structure and function of healthy SC, and in turn help us to understand the changes in
them upon exposure to weather conditions, cosmetic products (such as skin cleansing and
skin care products), and environmental pollutants.
Regular use of cosmetics and personal care products has become part of daily routine in the
modern era. Practices that started off as enhancing daily hygiene and beauty have evolved
into care and protection of skin, scalp, hair, and oral surfaces. Preventive care from harmful
effects arising from UV radiation and environmental pollutants have added another
dimension to the care aspects of personal care products. Furthermore, with the anticipated
increase in the aging population that is expected to exceed over a billion people of 65+
years age, and with skin as a major conduit for infections for this population, protecting
the skin barrier with personal care products for the elderly has become an important part
of the wellness regime. Thus, cosmetic and personal care products have become part of the
wellness-healthcare continuum to ensure an improved quality of life and healthcare for all.
All these trends have increased the exposure of a wide variety of products and associated
chemicals daily, and therefore the safety and efficacy of cosmetic and personal care products
have become even more important than ever before.
While there is an increased use of personal care and cosmetics products for reasons discussed
above, there is also an increased awareness among consumers through social media about
various ingredients that are being used in daily use products. This trend is driving use of
more “naturals” and sustainable ingredients into the product line. The trend also extends to
“free-of” technologies such as sulfate-free, preservative-free, and fragrance-free products. These
changes offer interesting challenges and opportunities for formulators requiring increased
scrutiny of the alternatives for ensuring product efficacy, consumer desired sensory, and safety.
In this changing environment, it is important that the choice of ingredients for cosmetic and
personal care products are made based on a scientific understanding of their safety, efficacy,
and their effects on the SC barrier. In this review, interaction of common classes of ingredients
encountered in personal care products is reviewed and general guidelines for selection of
ingredients that will have minimal impact on the SC barrier are discussed. Specifically, the
discussion here will include surfactants, emulsifiers, emollients, occlusives, humectants, skin
benefit lipids, and penetration enhancers. In areas where recent reviews are available, the
discussion will be limited to general guidelines with appropriate literature references.
THE STRUCTURE OF HUMAN STRATUM CORNEUM
The SC, the uppermost layer of human skin, is the main barrier that prevents excessive
water loss from the body and prevents the penetration of chemicals, pollutants, and germs