Personal Care Products and Human Health

Personal Care Products and Human Health provides background, historical context and the latest research results on personal care products (PCPs) and their impact on human health and the environment. Sections provide an overview of the functions and mechanisms of action of components of personal care products, discuss environmental toxicology, outline the problems of contamination of water systems from increasing use of personal care products and the resulting toxicities to aquatic wildlife, and offer chapters written by specialists on different aspects of concern for the effects of excessive personal care product usage on human health.

This is a comprehensive reference for toxicologists, environment scientists and those interested in learning about the science behind personal care products and current concerns for environmental and human health.

• Provides an overview of the mechanisms of action of components used in personal care products
• Reviews environmental contamination arising from increasing use of personal care products
• Examines concerns for human health arising from increasing use of personal care products

• Language: English
• Publisher: Academic Press
• Release date: Aug 1, 2023
• ISBN: 9780323996853

Book preview

Preface

Philippa D. Darbre,     School of Biological Sciences, University of Reading, Reading, United Kingdom

In all aspects of our lives, balance is essential for our well-being. Biology testifies to a need for just the right amount of everything, not too much and not too little, and each at the appropriate time. Even the ancient Greek philosopher Socrates is credited with the statement of everything in moderation. The challenge is how to reconcile this ancient wisdom with a modern consumer economy of growth, and nowhere is this more evident than in the growing global markets of personal care products. These products have brought great benefit for personal grooming and personal hygiene, but their mass marketing is now resulting in an unprecedented increase in usage, in terms of greater amounts applied at each application, increased frequency of application, and greater numbers of similar products used simultaneously. This book questions how far such increasing usage can remain beneficial before adverse impacts become evident on human health.

Many studies are now being published in the scientific literature concerning the adverse impacts of ingredients of personal care products on environmental and human health, and this book aims to collate these studies into a textbook. This book begins with a first section reviewing the functions and mechanisms of action of the many different components used in the formulation of personal care products. The second section provides an overview of the extent of environmental contamination now arising from increasing use of these products as they are washed down the drains and into water systems across the world. The section then continues with a review of the consequent uptake of component chemicals into human tissues, focusing on studies where tissue concentrations reflect product usage. The third section provides a series of reviews of different aspects of concern for the effects of excessive personal care product usage on human health. This includes increasing prevalence of contact dermatitis and of respiratory irritation and sensitization. The consequences of incorporating endocrine disrupting chemicals into personal care products are reviewed in relation not only to female and male reproductive health but also to consequences of early life exposures which may disrupt hormonally regulated developmental processes. The potential for ingredients of personal care products to influence cancer development is discussed, together with more specific concerns that ingredients of underarm cosmetics may be driving the rising incidence of breast cancer. The final section discusses the legacy of weak regulatory controls on many of these products and the need for more robust regulation especially concerning the entry of complex chemical mixtures into human tissues, use of nanomaterials, and incorporation of cosmeceuticals.

I would like to thank all those who have contributed to this book, firstly and most especially the coauthors who have given of their time to contribute. I would also like to acknowledge the many scientists who have contributed to this field but who are too numerous to give sufficient credit in the references cited. From a personal perspective, I would like to thank my scientific colleagues who have guided and encouraged me along the way, members of charity organizations and media presenters who have challenged me out of my academic comfort zone, and members of the public who have taken the time to write to me with so much useful information. Finally, I would like to thank my family who have walked with me through all my years of research and through the writing of this book.

Section 1

Introduction to the constituents of PCPs and their functions

Outline

Chapter 1. Introduction to personal care products

Chapter 2. Overview of constituent compounds with concerns for adverse effects on human health

Chapter 3. Nanomaterials

Chapter 4. Cosmeceuticals

Chapter 5. Plant-based ingredients in personal care products

Chapter 1: Introduction to personal care products

Philippa D. Darbre     School of Biological Sciences, University of Reading, Reading, United Kingdom

Abstract

Personal care products are consumer goods designed for external use on the body for the purposes of beautification, grooming, and personal hygiene, and this chapter provides an introduction to the wide product applications and the many constituent ingredients. In the context of external use on the body, the main organ in contact with personal care products is the skin, but inhaled products may contact external surfaces of the respiratory system, oral products will contact the surfaces of the oral cavity, and other products will contact hair and external genitalia. The chapter outlines the many different components used as active or supporting ingredients including cleansers, surfactants, abrasives, conditioning and moisturizing agents, solvents, emulsifiers, spreading agents, penetration enhancers, plasticizers, thickening and stiffening agents, preservatives, deodorants, antiperspirant, dehydrators, ultraviolet filters, fragrance and fragrance fixers, coloring agents, biocides, and cosmeceuticals. Age- and gender-targeted products are outlined. Different product formats and packaging are described. The chapter ends with an outline acknowledging the many benefits of personal care products but warning of the need for vigilance against associated contact dermatitis and respiratory irritation together with arising concerns surrounding endocrine-disrupting properties of many of the constituent chemicals especially with regard to human reproductive health, metabolic health, early life exposures, and cancer.

Keywords

Baby care; Cosmetics; Make-up; Personal care products; Skin care; Toiletries

1.1 Introduction

1.2 Historical background

1.3 Sites of exposure of the human body to personal care products

1.3.1 Human skin

1.3.2 Human hair

1.3.3 Human respiratory system

1.3.4 Human oral cavity

1.3.5 Human female genitalia

1.4 Functional components of personal care products

1.4.1 Cleansers, surfactants, and abrasives

1.4.2 Conditioning and moisturizing agents

1.4.3 Solvents, emulsifiers, spreading agents, penetration enhancers, and plasticizers

1.4.4 Thickening and stiffening agents

1.4.5 Preservatives

1.4.6 Deodorants

1.4.7 Antiperspirant

1.4.8 Dehydrators

1.4.9 UV filters

1.4.10 Fragrance and fragrance fixers

1.4.11 Coloring agents

1.4.12 Biocides

1.5 Cosmeceuticals

1.6 Age- and gender-targeted products

1.7 Product format and packaging

1.8 Benefits, adverse effects, and arising concerns

1.9 Concluding comments

References

1.1. Introduction

Personal care products (PCPs) are consumer products designed for external use on the body for the purposes of beautification, grooming, and personal hygiene. They include a very diverse range of products used for skin and body care including cleansing, moisturizing, and conditioning. They include products for hair care, for shaving, for oral hygiene, for feminine hygiene, for removing body odors, and adding fragrance. They include an extensive range of cosmetics and make-up to improve or change appearance. They include suncare products and products for coloring such as hair dyes, nail varnishes, and tanning. Most are designed for use by adults, but there are also products sold for use by babies (baby care). Toiletries are considered products used just for cleaning or grooming and may include items as diverse as brushes, towels, shower caps, wet wipes, toilet paper, cotton swabs, toothbrush, nail files, nail scissors/clippers, and razors. Cosmeceuticals are a newer range of products with biologically active ingredients alleged to provide health benefits. The main types of PCPs are outlined in Fig. 1.1.

Cosmetics are used to enhance appearance by concealing blemishes, enhancing natural features, adding color, and ensuring fragrant smell. The U.S. Food and Drug Administration (FDA) defines a cosmetic as a product (excluding pure soap) intended to be applied to the human body for cleansing, beautifying, promoting attractiveness, or altering the appearance (FDA, 2022a). The European Union (EU) defines a cosmetic product as any substance or preparation intended to be placed in contact with the various external parts of the human body (epidermis, hair system, nails, lips and external genital organs) or with the teeth and the mucous membranes of the oral cavity with a view exclusively or mainly to cleaning them, perfuming them, changing their appearance and/or correcting body odors and/or protecting them or keeping them in good condition (EU, 1976). Make-up is a term usually associated with cosmetics applied to the face or the nails (Fig. 1.2) and includes a wide range of products to improve facial skin through cleansers, application of foundation, face creams or face powder and to color facial skin through highlighter, blush, face paints, or bronzer. It includes products to apply to the lips such as lipsticks, lip gloss, and lip balm. It includes products for the eyes including eye shadow, mascara, and eyeliner. It includes a range of products for the nails including nail polishes and polish removers. Finally, it includes perfumes and colognes designed to give a fragrant smell (Fig. 1.2).

Figure 1.1  Different types of personal care products: products designed for external use on the body for beautification, grooming, or personal hygiene.

1.2. Historical background

Cosmetics have been used for thousands of years through the history of civilization in every corner of the globe (Stewart, 2017) (Fig. 1.3). Their use is recorded in Ancient Egypt, in the Old Testament of the Bible, in Ancient Greece, in Ancient Rome, and throughout medieval Britain to current times. In Ancient Egypt, cosmetics were considered a marker of wealth and were believed to please the gods. The use of eyeliners is visible on many ancient Egyptian relics, and the use of kohl, rouge, white powders to lighten skin tone, and malachite eye shadow were all popular. The word cosmetic derives from the Greek word kosmeticos, and cosmetics were used throughout ancient Greece as a symbol of wealth and beauty. Biblical recordings of cosmetic use were less positive with cosmetic use associated by the prophet Jeremiah with vanity (Jeremiah ch4 v30) and by records of Queen Jezebel with wickedness (2Kings ch9 v30). The use of cosmetics was widespread in the Roman Empire, despite negative sentiments by some Roman and Christian writers of the time, and antiaging recipes were thought to have been inspired by the goddess Venus. Eastern cultures have always embraced the use of floral cosmetics, and the ancient traditions of face-painting by actors continue today in the traditional Chinese operas.

Figure 1.2  Different types of cosmetics or make-up applied mainly to the face and nails which are used to enhance attractiveness and which are used by actors to improve or change appearance.

Figure 1.3  Historical timeline of the use of personal care products from Ancient Egypt to modern global mass marketing.

During the European Middle Ages, light skin was associated with high social standing and practices of whitening facial skin were popular. Indeed, Queen Elizabeth 1 is said to have used cosmetics with arsenic to whiten her face. In the 19th century, Queen Victoria's public declarations of dislike of cosmetics together with a growing concern of the toxicity of some preparations caused cosmetic use to decline, and this continued through the days of austerity of the first and second world wars. However, after the end of the Second World War, a new brighter era dawned signaling a departure from the deprived war days and a more carefree tomorrow. Rising disposable incomes coincided with introduction of the silver screens and the rise of celebrity culture, which together generated the perceived necessity for a perfect body image. This was further fueled by the influences of mass advertising and mass marketing of consumer products through printed, internet, and screen media, and has been further exacerbated over recent decades by social media. This has led to a whole new era for expansion of the use not only of cosmetics but also of wider PCPs.

1.3. Sites of exposure of the human body to personal care products

PCPs are designed for use on external parts of the body, and therefore the major surface body area in contact with these products is skin, from which internalization of chemical components may occur though dermal absorption processes. However, ranges of hair-care products will be in contact not only with hair on the head but also with the hair shaft and hair follicles across the body surface. Any components of PCPs which can get into the air may be inhaled, and therefore come in contact with external surfaces of the respiratory tract. Oral care products will contact the mucous membranes of the oral cavity, the teeth, and the lips. Feminine hygiene products are designed for use in female genital regions and will therefore contact external surfaces of the vulval and vaginal regions. Most exposure to components of PCPs is therefore through surface absorption of chemical ingredients, but it is also possible that there can be oral ingestion of products used in and around the mouth, or transferred into the mouth on fingers or hands or from air. Relevant structural features of the external surfaces of these body parts are outlined in the sections below.

1.3.1. Human skin

In the context of external use on the body, the main organ in contact with PCPs is the skin. The skin covers the entire body surface and functions in protecting the body from external damage (Kupper and Fulbrigge, 2004; Basler et al., 2016; Juranova et al., 2017; Dąbrowska et al., 2018; Roger et al., 2019) including through immune surveillance mechanisms (Kupper and Fulbrigge, 2004). The skin also plays an essential role in maintaining body homeostasis by regulating body temperature and preventing water and electrolyte loss (Basler et al., 2016; Dąbrowska et al., 2018; Kissel et al., 2018). Therefore, keeping the skin healthy and in good condition is an essential component of good health for the entire body, and PCPs are formulated with this focus. Moisturizing and conditioning products are designed to help keep the skin hydrated. When damaged, the skin can repair itself, but some PCPs are formulated to help with wound healing. The large surface area of the skin allows for dirt to accumulate and for attack by pathogens, and therefore PCPs play a major role in cleansing and personal hygiene.

Human skin is made up of three layers: the epidermis, the dermis, and the hypodermis (Fig. 1.4). The epidermis is the outermost layer of skin and is composed itself also of several layers with the stratum corneum being the outermost protective layer, followed by the stratum granulosum, the stratum spinosum, and the stratum basale. It is separated from the dermis below by a basement membrane. Proliferative cells (keratinocytes and their stem cells) in the stratum basale gradually migrate up through the different layers of the skin, becoming increasingly differentiated and filled with keratin. As they reach the stratum corneum, they have become terminally differentiated keratinized cells which then slough off at the skin surface. This outermost layer of keratinized keratinocytes, termed corneocytes, provides the protection for the body against loss of water, loss of heat, physical damage, and invasion by pathogens.

Figure 1.4  Diagrammatic illustration of the structure of human skin showing the many layers of the outer epidermis, together with the components making up the dermis and the lower hypodermis. Diagram has been adapted from Salvioni et al. (2021).

The dermis is the layer of skin beneath the basal membrane of the epidermis and contains connective tissue to cushion the body from external stresses and strains. It contains many nerve endings to provide the sensations of touch and heat. It contains hair follicles, sweat glands, sebaceous glands, apocrine glands, lymphatic vessels, and blood vessels. The superficial area closer to the epidermis is termed the papillary region, and the deeper area is the reticular region. The papillary region is composed of areolar connective tissue in finger-like projections termed papillae which project into the epidermis to provide a strong connection between the two layers. In palms of hands, fingers, soles of feet, and toes, the papillae cause contours on the surface of the skin surface which are unique to each person and allow fingerprints and footprints to be used for personal identification. The reticular region is a thick connective tissue layer with high concentrations of collagen and elastin giving strength and elasticity to the dermis. The hypodermis lies below the dermis and contains adipose tissue as well as connective tissue and serves to attach the skin to underlying bone and muscle of the body.

The skin is subject to damage by environmental radiation, and skin pigmentation helps to absorb untoward rays, especially in the ultraviolet (UV) range, and provide protection. Melanin is produced in skin cells termed melanocytes located in the basal layer of the epidermis. Within these cells, the melanin is synthesized, stored, and transported within organelles termed melanosomes. Melanogenesis can be increased by exposure to UV radiation, and the quantity and distribution of melanosomes within the melanocytes and the melanocytes within the skin can change skin color and protect against radiation damage. Suncare products are designed to provide additional protection to skin exposed to excessive sunlight.

The skin also contains a system for robust immune surveillance using both the innate and adaptive immune systems (Kupper and Fulbrigge, 2004). In the innate immune response, epithelial cell damage (physical, chemical agents, or pathogens) can lead to the release of primary cytokines and activation of both skin cells (keratinocytes and fibroblasts) and resident innate immune cells (Langerhans cells, dermal dendritic cells, and mast cells) stimulating downstream activation cascades (Kupper and Fulbrigge, 2004). The adaptive immune system is based on T cells and B cells that express antigen-specific receptors and provides a memory of previous encounters. At the skin surface, primary immune surveillance brings together antigen-presenting cells and naïve T cells in the microenvironment of skin-draining lymph nodes. Secondary responses involve production and distribution of antigen-specific effector memory T cells expressing homing receptors which direct their migration to the tissue where the antigen was encountered. Tertiary immune surveillance then encompasses the long-term elements of the acquired immune response including production of central memory and effector cells which can be directed to tissues other than the site of primary exposure (Kupper and Fulbrigge, 2004). Through these mechanisms, chemical components of PCPs can lead to both irritant (using the innate immune system) and allergenic (using the adaptive immune system) responses in the skin or other epithelial surfaces of the body which is discussed in more detail in Chapter 8.

1.3.2. Human hair

Hair has always played a major role in society. In women, a head of hair has been associated with beauty and youthfulness, described in the bible to be the crowning glory of a woman (1Corinthians, ch 11, v15). In men, it has been associated with masculinity and virility, and biblically cutting of Samson's hair is reported to have drained his strength (Judges ch 16, v17-18). Hair fashions vary in different cultures and over different time periods, but styling and coloring of hair on the head have always played a central role in society. Hair care products focus therefore on keeping hair in good condition and in offering appropriate cultural changes. Body hair image has also changed over time and culture, and modern products offer ways to manage or remove body hair as required.

Hair derives from hair follicles in the dermis and is composed of keratinized skin cells (Fig. 1.5). The follicles maintain stem cells which allow both for growth of hair and for regrowth of skin cells after a wound (Krause and Foitzik, 2006). The hair shaft is the hard filamentous strand that extends beyond the skin surface and is divided into four main structures, cuticle, cortex, intercellular material, and medulla (Wagner et al., 2007). The cuticle is the outer covering and consists of layers of flat thin cells. The outer surface is covered with lipid to allow the hair to repel water, and glands of the hair follicle produce a fatty secretion to keep the hair lubricated. The highly structured cortex lies below, providing mechanical strength and containing melanin which colors the hair according to number, distribution, and types of melanin granules. The medulla is an unstructured area at the center of the fiber.

Figure 1.5  Diagrammatic illustration of the structure of a human hair follicle, showing the layers of the hair shaft protruding above the skin surface and the arrangements of the hair follicle below the skin surface.

In covering the skin, hair functions in thermoregulation, in physical protection, and in protection from UV-derived sunlight. On the head, hair has always been considered as an adornment. Attitudes toward hair in terms of hairstyles and hair removal vary widely across different cultures, and products can aid to condition hair, to straighten hair, to curl hair, to color hair, and to remove hair at specified locations.

1.3.3. Human respiratory system

PCPs contain many volatile and semivolatile organic compounds which may become distributed between the gaseous and particulate phases of air (Anderson et al., 2012). Furthermore, some products are formulated in aerosol, spray formats, and powder forms which may result in pollution of indoor air spaces (Darbre, 2018). As a consequence, this may result in inhalation, and thereby exposure of the epithelial linings of the airways and lung tissues of the respiratory tract. The structure of the human respiratory tract and consequences for exposures to PCPs are discussed in detail in Chapter 9.

1.3.4. Human oral cavity

The oral cavity or mouth, bounded by the lips and ending at the throat, is important for tasks of ingesting food, breathing, and speaking. It is lined by mucous membranes and includes structures such as the teeth, gums, tongue, and salivary glands. It may be exposed to PCPs from air inhaled via the mouth and from the use of oral care products. Oral care products are designed to help in oral and dental hygiene (toothpastes, brushes, and mouthwashes) and beautification (tooth whiteners and lip products), from which some ingredients may be absorbed through the lining membranes of the cavity, while others may be ingested from the digestive tract through swallowing.

1.3.5. Human female genitalia

Feminine hygiene products are PCPs designed to cleanse and control discharges around the vulval and vaginal regions, especially during menstruation. They contain many chemical ingredients which will contact the external genitalia, including vulva and clitoris, and/or the internal genitalia, most immediately the vagina and cervix but may also reach beyond to the uterus, Fallopian tubes, and ovaries.

1.4. Functional components of personal care products

A PCP is composed of one or more active ingredients together with other ingredients that form the base or vehicle for presentation of the product (Fig. 1.6). The vehicle is formulated to make the product suitable for efficient transport of the active ingredient to the target site, and to ensure the active ingredient remains in the site of action for the period of time needed to achieve the required effect (Buchmann, 2005). Furthermore, the vehicle must support the physical, chemical, and microbiological stability not only of the active ingredient but also of the entire product. Although the active ingredient may be obvious in some cases, there is often a blurring between active and supporting ingredient. The many different ingredients used as active and supporting ingredients are outlined below and summarized in Fig. 1.6.

1.4.1. Cleansers, surfactants, and abrasives

Cleansers are used to clean surfaces which in the context of PCPs means the outer surfaces of the body. This may include removal of not only environmental dirt and microorganisms but also dead cells, body oils, body sweat, and any residues of previously applied products. Many different types of cleansers are used in PCPs, often soaps or detergents. Soaps are gentler cleansers made of natural ingredients such as oils and fats, while detergents are stronger cleansers containing synthetic surfactants.

Figure 1.6  An outline of the many functional components of personal care products.

Surfactants are surface-active agents that lower the surface tension between two liquids, enabling them to act as detergents to clean the skin and hair by removing unwanted oils and dirt. Surfactants are usually organic molecules that are amphiphilic, containing both hydrophobic and hydrophilic groups, allowing them to diffuse in water but absorb at the interface between water and oil of the skin/hair. There are nonionic (stearyl alcohol and cetearyl alcohol), anionic (sodium lauryl sulfate and sodium stearate), cationic (quaternary ammonium salts), and amphoteric (betaines) surfactants (Prieto-Blanco et al., 2018). Nonylphenol ethoxylates and nonylphenol are nonionic surfactants also used in PCPs.

For more intensive cleaning, abrasives can be used to aid in the removal of dead cells (exfoliating) and polishing the surface. Mechanical processes involve scrubbing the skin with an abrasive material such as microfiber cloths, adhesive exfoliation sheets, microbead scrubs, pumice (volcanic rock) or sponges, loofahs, and brushes. Chemical exfoliation (chemical peeling) can be achieved with a wide range of substances including alpha-hydroxy acids, beta-hydroxy acids, polyhydroxy acids, or enzymes that weaken cell–cell adhesion (e.g., trypsin and collagenase). Some methods of hair removal can also exfoliate the skin. Waxing plucks hair out of the skin by a mechanical process and can also exfoliate. In wet shaving, moving a shaving brush over the skin surface may exfoliate. Chemical depilatory methods using calcium hydroxide or thioglycolic acid may also act as a chemical exfoliant.

1.4.2. Conditioning and moisturizing agents

A conditioner is a substance that improves the quality of the skin. In cosmetics, these are often moisturizers added to improve hydration of the skin. When water is lost from the stratum corneum more rapidly than it is supplied by the lower layers of the dermis, the skin becomes dehydrated, losing its flexibility and attaining a rough surface which is more vulnerable to cracking. Moisturizing agents are designed to rehydrate the skin. Emollients are hydrophobic molecules that can coat the skin to provide an occlusive surface that help to prevent further loss of moisture and are often oils, fats, waxes, or silicones. Humectants can hydrogen bond with water in the skin to reduce water loss and are typically polyols or glycols. Other conditioners include fatty alcohols which can provide a hydrophobic coating to the skin, and cationic surfactants that can provide protective films on skin and hair.

1.4.3. Solvents, emulsifiers, spreading agents, penetration enhancers, and plasticizers

Solvents are an essential component of PCPs because they act as the carriers to dissolve the contained ingredients. Commonly used solvents include water (aqua), vegetable or animal oils, silicones, or alcohols.

Emulsifiers are compounds added to aid in the mixing of liquids with different properties which would otherwise be immiscible (unmixable). Since oils and fats are often added together with water into PCPs, emulsifiers are needed to produce a homogenous product of oil-in-water or water-in-oil (Costa et al., 2019). The use of oil-in-water-in-oil double emulsions can offer improvements in emulsification (Zhi et al., 2022), as can also decreasing the particle size to within the nanoscale in microemulsions (Szumala and Macierzanke, 2022).

Spreading agents are substances that lower the surface tension and help to cover surfaces of the skin and hair in a more even manner. Penetration enhancers are substances which help ingredients of a PCP to penetrate beyond the outer surface of the skin. Such agents act to decrease the barrier resistance of the skin by interacting with components of the skin such as the intercellular lipid matrix, the intracellular keratin domains, or the desmosomal connections between corneocytes (Williams and Barry, 2004). The use of lipid- and surfactant-based vesicle structures can also increase skin penetration (see Chapter 3) (Ashtiani et al., 2016).

Plasticizers are added as low molecular weight compounds to polymer solutions to increase plasticity and to decrease viscosity. They can be used as solvents, penetration enhancers, and fragrance fixers. They can help to restore flexibility to the skin. Plasticizers make polymer solutions more suitable for the application of films such as in nail care products.

1.4.4. Thickening and stiffening agents

Thickening and stiffening agents are added to PCPs to provide various degrees of viscosity to the product and to stabilize suspensions/emulsions. Formulations can be thickened with soluble swellable polymers or colloidal structures. Gelling agents may be used to form a gel by dissolving in the liquid phase as a colloid mixture with a weakly cohesive internal structure. Other thickeners may act through discrete particles adhering or interlocking. Natural polymers include starches, polysaccharides, and proteins. Waxes include long-chain hydrocarbons, fatty alcohols, triglycerides, and esters. Viscosifying particles include silicone elastomers, aerogels, fumed silica, and modified clays. Oil-soluble polymers include polyethylene glycol, ethylene-acrylic copolymers, synthetic wax, dextran fatty esters, and modified acrylates. Polymers of acrylic acid, methacrylic acid, or one of their esters are used in hair styling products to stiffen and bulk the hair during setting and to protect it from humidity (Jachowicz and Yao,