Titanium dioxide

Titanium dioxide (TiO₂) and zinc oxide (ZnO) in large-particle form have long been used in various sunscreens to protect the skin by reflecting or physically blocking ultraviolet (UV) radiation. In recent years, TiO₂ as well as ZnO nanoparticles have been incorporated into sunscreens and cosmetics to act as a UV shield. They have been shown to be effective barriers against UV-induced damage, and yield stronger protection against UV insult, while leaving less white residue, than previous generations of the physical sunblocks.

However, some data suggest that in nanoparticle form, TiO₂ and ZnO absorb UV radiation, leading to photocatalysis and the release of reactive oxygen species (Australas. J. Dermatol. 2011;52:1-6). This column will focus primarily on the safety of TiO₂ in nanoparticle form.

While numerous studies examine both TiO₂ and ZnO, the primary inorganic sunscreens, the sheer number of separate investigations warrants individual articles, and ZnO was addressed in previous columns. Briefly, though, TiO₂ is more photoactive and exhibits a higher refractive index in visible light than ZnO (J. Am. Acad. Dermatol. 1999;40:85-90); therefore, TiO₂ appears whiter and is more difficult to incorporate into transparent products.

A 2011 study by Kang et al. showed that TiO₂ nanoparticles, but not normal-sized TiO₂, and UVA synergistically foster rapid production of reactive oxygen species and breakdown of mitochondrial membrane potential, leading to apoptosis, and that TiO₂ nanoparticles are more phototoxic than larger ones (Drug Chem. Toxicol. 2011;34:277-84).

However, also in 2011, Tyner et al. investigated the effects of nanoscale TiO₂ use on UV attenuation in simple to complex sunscreen products. They found that barrier function was diminished by none of the formulations, and that optimal UV attenuation resulted when TiO₂ particles were stabilized with a coating and evenly dispersed. The researchers concluded that nanoscale TiO₂ is nontoxic and may impart greater efficacy (Int. J. Cosmet. Sci. 2011;33:234-44).

In vitro and in vivo studies

In 2010, Tiano et al. evaluated five modified TiO₂ particles, developed and marketed for sunscreens. They used different in vitro models, including cultured human skin fibroblasts, to determine potential photocatalytic effects after UVA exposure. The investigators found that the kind of modification to and crystal form of the TiO₂ nanoparticle influences its ability to augment or reduce DNA damage, increase or decrease intracellular reactive oxygen species, diminish cell viability, and promote other effects of photocatalysis. In particular, they noted that the anatase crystal form of TiO₂ retained photocatalytic activity. The authors suggested that while the debate continues over the penetration of nanosized TiO₂ into the viable epidermis, their results help elucidate the potential effects of TiO₂ particles at the cellular level (Free Radic. Biol. Med. 2010;49:408-15).

A 2010 study by Senzui et al. using in vitro intact, stripped, and hair-removed skin of Yucatan micropigs to test the skin penetration of four different types of rutile (the most natural form of) TiO₂ (two coated, two uncoated) revealed no penetration of TiO₂ type in intact and stripped skin. The concentration of titanium in skin was significantly higher when one of the coated forms was applied on hair-removed skin, with titanium penetrating into vacant hair follicles (greater than 1 mm below the skin surface), but not into dermis or viable epidermis (J. Toxicol. Sci. 2010;35:107-13).

Animal studies

In 2009, the Food and Drug Administration Center for Drug Evaluation and Research worked with the National Center for Toxicology Research using minipigs and four sunscreen formulations to determine whether nanoscale TiO₂ can penetrate intact skin. Their use of scanning electron microscopy and x-ray diffraction revealed that TiO₂ particles were the same size as that observed for the raw materials, implying that the formulation process influenced neither the size nor the shape of TiO₂ particles (Drug Dev. Ind. Pharm. 2009;35:1180-9).

In 2010, Sadrieh et al. performed a study of the dermal penetration of three TiO₂ particles: uncoated submicrometer-sized, uncoated nano-sized, and dimethicone/methicone copolymer-coated nanosized. The investigators applied 5% by weight of each of the types of particles in a sunscreen on minipigs and found no significant penetration into intact normal epidermis (Toxicol. Sci. 2010;115(1):156-66).

In 2011, Furukawa et al. studied the postinitiation carcinogenic potential of coated and uncoated TiO₂ nanoparticles in a two-stage skin carcinogenesis model using 7-week-old CD1 (ICR) female mice. They found that application of coated and uncoated nanoparticles after initiation and promotion with 7,12-dimethylbenz[a]anthracene and 12-O-tetradecanoylphorbol 13-acetate at doses of up to 20 mg/mouse failed to augment nodule development. The investigators concluded that TiO₂ nanoparticles do not exhibit postinitiation potential for mouse skin carcinogenesis (Food Chem. Toxicol. 2011;49(4):744-9).

Human data

Given the persistent concerns about possible side effects of coated TiO₂ and ZnO nanoparticles used in physical sun blockers, Filipe et al., in 2009, assessed the localization and potential skin penetration of TiO₂ and ZnO nanoparticles dispersed in three sunscreen formulations, under realistic in vivo conditions in normal and altered skin. The investigators examined a test hydrophobic formulation containing coated 20-nm TiO₂ nanoparticles and two commercially available sunscreen formulations containing TiO₂ alone or in combination with ZnO, with respect to how consumers actually used sunscreens compared with the recommended standard condition for the sun protection factor test. They found that traces of the physical blockers could be detected only at the skin surface and uppermost area of the stratum corneum in normal human skin after a 2-hour exposure. After 48 hours of exposure, layers deeper than the stratum corneum contained no detectable TiO₂ or ZnO nanoparticles. While preferential deposition of the nanoparticles in the openings of pilosebaceous follicles was noted, no penetration into viable skin tissue was observed. The investigators concluded that significant penetration of TiO₂ or ZnO nanoparticles into keratinocytes is improbable (Skin Pharmacol. Physiol. 2009;22:266-75).

The weight of evidence

Current evidence suggests minimal risks to human health from the use of TiO₂ or ZnO nanoparticles at concentrations up to 25% in cosmetic preparations or sunscreens, according to Schilling et al., regardless of coatings or crystalline structure. In a safety review of these ingredients, they noted that these nanoparticles formulated in topical products occur as aggregates of primary particles 30-150 nm in size, and bond in such a way that renders them impervious to the force of product application. Thus their structure remains unaffected, and no primary particles are released. The authors also noted that nanoparticles exhibit equivalence with larger particles in terms of distribution and duration and, therefore, recognition and elimination from the body (Photochem. Photobiol. Sci. 2010;9:495-509).

But in 2011, Tran and Salmon, in light of findings that nanoparticles may penetrate the stratum corneum under certain conditions, considered the possible photocarcinogenic results of nanoparticle sunscreens. They noted, though, that most such results were obtained through the use of animal skin models, not investigations with human skin (Australas. J. Dermatol. 2011;52:1-6). To this point, the weight of evidence appears to show that such TiO₂ nanoparticles are safe when applied to intact human skin (Semin. Cutan. Med. Surg. 2011;30:210-13).

In response to the increased scrutiny and concern exhibited by the general public and government agencies regarding the safety of TiO₂ and ZnO nanoparticles, Newman et al. reviewed the literature and position statements from 1980 to 2008 to ascertain and describe the use, safety, and regulatory state of such ingredients in sunscreens. They found no evidence of significant penetration deeper than the stratum corneum of TiO₂ and ZnO nanoparticles, but caution that additional studies simulating real-world conditions (i.e., sunburned skin and under UV exposure) are necessary (J. Am. Acad. Dermatol. 2009;61:685-92).

Conclusion

Titanium dioxide is a well-established, safe, and effective physical sunblock. Nanotechnology has introduced some cause for concern regarding its use in physical sunblocks. In particular, evidence suggesting that photoexcitation of TiO₂ nanoparticles leads to the generation of reactive oxygen species that damage DNA, potentially launching a cascade of adverse events, has prompted investigations into the safety of TiO₂ in nanoparticle form. However, to date, multiple studies suggest that TiO₂ nanoparticles do not penetrate or are highly unlikely to penetrate beyond the stratum corneum.

Dr. Baumann is chief executive officer of the Baumann Cosmetic & Research Institute in Miami Beach. She founded the cosmetic dermatology center at the University of Miami in 1997. Dr. Baumann wrote the textbook "Cosmetic Dermatology: Principles and Practice" (McGraw-Hill, 2002), and a book for consumers, "The Skin Type Solution" (Bantam, 2006). Dr. Baumann has received funding for clinical grants from Allergan, Aveeno, Avon Products, Galderma, Mary Kay, Medicis Pharmaceuticals, Neutrogena, Philosophy, Stiefel, Topix Pharmaceuticals, and Unilever.

Titanium dioxide (TiO₂) and zinc oxide (ZnO) in large-particle form have long been used in various sunscreens to protect the skin by reflecting or physically blocking ultraviolet (UV) radiation. In recent years, TiO₂ as well as ZnO nanoparticles have been incorporated into sunscreens and cosmetics to act as a UV shield. They have been shown to be effective barriers against UV-induced damage, and yield stronger protection against UV insult, while leaving less white residue, than previous generations of the physical sunblocks.

However, some data suggest that in nanoparticle form, TiO₂ and ZnO absorb UV radiation, leading to photocatalysis and the release of reactive oxygen species (Australas. J. Dermatol. 2011;52:1-6). This column will focus primarily on the safety of TiO₂ in nanoparticle form.

While numerous studies examine both TiO₂ and ZnO, the primary inorganic sunscreens, the sheer number of separate investigations warrants individual articles, and ZnO was addressed in previous columns. Briefly, though, TiO₂ is more photoactive and exhibits a higher refractive index in visible light than ZnO (J. Am. Acad. Dermatol. 1999;40:85-90); therefore, TiO₂ appears whiter and is more difficult to incorporate into transparent products.

A 2011 study by Kang et al. showed that TiO₂ nanoparticles, but not normal-sized TiO₂, and UVA synergistically foster rapid production of reactive oxygen species and breakdown of mitochondrial membrane potential, leading to apoptosis, and that TiO₂ nanoparticles are more phototoxic than larger ones (Drug Chem. Toxicol. 2011;34:277-84).

However, also in 2011, Tyner et al. investigated the effects of nanoscale TiO₂ use on UV attenuation in simple to complex sunscreen products. They found that barrier function was diminished by none of the formulations, and that optimal UV attenuation resulted when TiO₂ particles were stabilized with a coating and evenly dispersed. The researchers concluded that nanoscale TiO₂ is nontoxic and may impart greater efficacy (Int. J. Cosmet. Sci. 2011;33:234-44).

In vitro and in vivo studies

In 2010, Tiano et al. evaluated five modified TiO₂ particles, developed and marketed for sunscreens. They used different in vitro models, including cultured human skin fibroblasts, to determine potential photocatalytic effects after UVA exposure. The investigators found that the kind of modification to and crystal form of the TiO₂ nanoparticle influences its ability to augment or reduce DNA damage, increase or decrease intracellular reactive oxygen species, diminish cell viability, and promote other effects of photocatalysis. In particular, they noted that the anatase crystal form of TiO₂ retained photocatalytic activity. The authors suggested that while the debate continues over the penetration of nanosized TiO₂ into the viable epidermis, their results help elucidate the potential effects of TiO₂ particles at the cellular level (Free Radic. Biol. Med. 2010;49:408-15).

A 2010 study by Senzui et al. using in vitro intact, stripped, and hair-removed skin of Yucatan micropigs to test the skin penetration of four different types of rutile (the most natural form of) TiO₂ (two coated, two uncoated) revealed no penetration of TiO₂ type in intact and stripped skin. The concentration of titanium in skin was significantly higher when one of the coated forms was applied on hair-removed skin, with titanium penetrating into vacant hair follicles (greater than 1 mm below the skin surface), but not into dermis or viable epidermis (J. Toxicol. Sci. 2010;35:107-13).

Animal studies

In 2009, the Food and Drug Administration Center for Drug Evaluation and Research worked with the National Center for Toxicology Research using minipigs and four sunscreen formulations to determine whether nanoscale TiO₂ can penetrate intact skin. Their use of scanning electron microscopy and x-ray diffraction revealed that TiO₂ particles were the same size as that observed for the raw materials, implying that the formulation process influenced neither the size nor the shape of TiO₂ particles (Drug Dev. Ind. Pharm. 2009;35:1180-9).

In 2010, Sadrieh et al. performed a study of the dermal penetration of three TiO₂ particles: uncoated submicrometer-sized, uncoated nano-sized, and dimethicone/methicone copolymer-coated nanosized. The investigators applied 5% by weight of each of the types of particles in a sunscreen on minipigs and found no significant penetration into intact normal epidermis (Toxicol. Sci. 2010;115(1):156-66).

In 2011, Furukawa et al. studied the postinitiation carcinogenic potential of coated and uncoated TiO₂ nanoparticles in a two-stage skin carcinogenesis model using 7-week-old CD1 (ICR) female mice. They found that application of coated and uncoated nanoparticles after initiation and promotion with 7,12-dimethylbenz[a]anthracene and 12-O-tetradecanoylphorbol 13-acetate at doses of up to 20 mg/mouse failed to augment nodule development. The investigators concluded that TiO₂ nanoparticles do not exhibit postinitiation potential for mouse skin carcinogenesis (Food Chem. Toxicol. 2011;49(4):744-9).

Human data

Given the persistent concerns about possible side effects of coated TiO₂ and ZnO nanoparticles used in physical sun blockers, Filipe et al., in 2009, assessed the localization and potential skin penetration of TiO₂ and ZnO nanoparticles dispersed in three sunscreen formulations, under realistic in vivo conditions in normal and altered skin. The investigators examined a test hydrophobic formulation containing coated 20-nm TiO₂ nanoparticles and two commercially available sunscreen formulations containing TiO₂ alone or in combination with ZnO, with respect to how consumers actually used sunscreens compared with the recommended standard condition for the sun protection factor test. They found that traces of the physical blockers could be detected only at the skin surface and uppermost area of the stratum corneum in normal human skin after a 2-hour exposure. After 48 hours of exposure, layers deeper than the stratum corneum contained no detectable TiO₂ or ZnO nanoparticles. While preferential deposition of the nanoparticles in the openings of pilosebaceous follicles was noted, no penetration into viable skin tissue was observed. The investigators concluded that significant penetration of TiO₂ or ZnO nanoparticles into keratinocytes is improbable (Skin Pharmacol. Physiol. 2009;22:266-75).

The weight of evidence

Current evidence suggests minimal risks to human health from the use of TiO₂ or ZnO nanoparticles at concentrations up to 25% in cosmetic preparations or sunscreens, according to Schilling et al., regardless of coatings or crystalline structure. In a safety review of these ingredients, they noted that these nanoparticles formulated in topical products occur as aggregates of primary particles 30-150 nm in size, and bond in such a way that renders them impervious to the force of product application. Thus their structure remains unaffected, and no primary particles are released. The authors also noted that nanoparticles exhibit equivalence with larger particles in terms of distribution and duration and, therefore, recognition and elimination from the body (Photochem. Photobiol. Sci. 2010;9:495-509).

But in 2011, Tran and Salmon, in light of findings that nanoparticles may penetrate the stratum corneum under certain conditions, considered the possible photocarcinogenic results of nanoparticle sunscreens. They noted, though, that most such results were obtained through the use of animal skin models, not investigations with human skin (Australas. J. Dermatol. 2011;52:1-6). To this point, the weight of evidence appears to show that such TiO₂ nanoparticles are safe when applied to intact human skin (Semin. Cutan. Med. Surg. 2011;30:210-13).

In response to the increased scrutiny and concern exhibited by the general public and government agencies regarding the safety of TiO₂ and ZnO nanoparticles, Newman et al. reviewed the literature and position statements from 1980 to 2008 to ascertain and describe the use, safety, and regulatory state of such ingredients in sunscreens. They found no evidence of significant penetration deeper than the stratum corneum of TiO₂ and ZnO nanoparticles, but caution that additional studies simulating real-world conditions (i.e., sunburned skin and under UV exposure) are necessary (J. Am. Acad. Dermatol. 2009;61:685-92).

Conclusion

Titanium dioxide is a well-established, safe, and effective physical sunblock. Nanotechnology has introduced some cause for concern regarding its use in physical sunblocks. In particular, evidence suggesting that photoexcitation of TiO₂ nanoparticles leads to the generation of reactive oxygen species that damage DNA, potentially launching a cascade of adverse events, has prompted investigations into the safety of TiO₂ in nanoparticle form. However, to date, multiple studies suggest that TiO₂ nanoparticles do not penetrate or are highly unlikely to penetrate beyond the stratum corneum.

Dr. Baumann is chief executive officer of the Baumann Cosmetic & Research Institute in Miami Beach. She founded the cosmetic dermatology center at the University of Miami in 1997. Dr. Baumann wrote the textbook "Cosmetic Dermatology: Principles and Practice" (McGraw-Hill, 2002), and a book for consumers, "The Skin Type Solution" (Bantam, 2006). Dr. Baumann has received funding for clinical grants from Allergan, Aveeno, Avon Products, Galderma, Mary Kay, Medicis Pharmaceuticals, Neutrogena, Philosophy, Stiefel, Topix Pharmaceuticals, and Unilever.

Titanium dioxide (TiO₂) and zinc oxide (ZnO) in large-particle form have long been used in various sunscreens to protect the skin by reflecting or physically blocking ultraviolet (UV) radiation. In recent years, TiO₂ as well as ZnO nanoparticles have been incorporated into sunscreens and cosmetics to act as a UV shield. They have been shown to be effective barriers against UV-induced damage, and yield stronger protection against UV insult, while leaving less white residue, than previous generations of the physical sunblocks.

However, some data suggest that in nanoparticle form, TiO₂ and ZnO absorb UV radiation, leading to photocatalysis and the release of reactive oxygen species (Australas. J. Dermatol. 2011;52:1-6). This column will focus primarily on the safety of TiO₂ in nanoparticle form.

While numerous studies examine both TiO₂ and ZnO, the primary inorganic sunscreens, the sheer number of separate investigations warrants individual articles, and ZnO was addressed in previous columns. Briefly, though, TiO₂ is more photoactive and exhibits a higher refractive index in visible light than ZnO (J. Am. Acad. Dermatol. 1999;40:85-90); therefore, TiO₂ appears whiter and is more difficult to incorporate into transparent products.

A 2011 study by Kang et al. showed that TiO₂ nanoparticles, but not normal-sized TiO₂, and UVA synergistically foster rapid production of reactive oxygen species and breakdown of mitochondrial membrane potential, leading to apoptosis, and that TiO₂ nanoparticles are more phototoxic than larger ones (Drug Chem. Toxicol. 2011;34:277-84).

However, also in 2011, Tyner et al. investigated the effects of nanoscale TiO₂ use on UV attenuation in simple to complex sunscreen products. They found that barrier function was diminished by none of the formulations, and that optimal UV attenuation resulted when TiO₂ particles were stabilized with a coating and evenly dispersed. The researchers concluded that nanoscale TiO₂ is nontoxic and may impart greater efficacy (Int. J. Cosmet. Sci. 2011;33:234-44).

In vitro and in vivo studies

In 2010, Tiano et al. evaluated five modified TiO₂ particles, developed and marketed for sunscreens. They used different in vitro models, including cultured human skin fibroblasts, to determine potential photocatalytic effects after UVA exposure. The investigators found that the kind of modification to and crystal form of the TiO₂ nanoparticle influences its ability to augment or reduce DNA damage, increase or decrease intracellular reactive oxygen species, diminish cell viability, and promote other effects of photocatalysis. In particular, they noted that the anatase crystal form of TiO₂ retained photocatalytic activity. The authors suggested that while the debate continues over the penetration of nanosized TiO₂ into the viable epidermis, their results help elucidate the potential effects of TiO₂ particles at the cellular level (Free Radic. Biol. Med. 2010;49:408-15).

A 2010 study by Senzui et al. using in vitro intact, stripped, and hair-removed skin of Yucatan micropigs to test the skin penetration of four different types of rutile (the most natural form of) TiO₂ (two coated, two uncoated) revealed no penetration of TiO₂ type in intact and stripped skin. The concentration of titanium in skin was significantly higher when one of the coated forms was applied on hair-removed skin, with titanium penetrating into vacant hair follicles (greater than 1 mm below the skin surface), but not into dermis or viable epidermis (J. Toxicol. Sci. 2010;35:107-13).

Animal studies

In 2009, the Food and Drug Administration Center for Drug Evaluation and Research worked with the National Center for Toxicology Research using minipigs and four sunscreen formulations to determine whether nanoscale TiO₂ can penetrate intact skin. Their use of scanning electron microscopy and x-ray diffraction revealed that TiO₂ particles were the same size as that observed for the raw materials, implying that the formulation process influenced neither the size nor the shape of TiO₂ particles (Drug Dev. Ind. Pharm. 2009;35:1180-9).

In 2010, Sadrieh et al. performed a study of the dermal penetration of three TiO₂ particles: uncoated submicrometer-sized, uncoated nano-sized, and dimethicone/methicone copolymer-coated nanosized. The investigators applied 5% by weight of each of the types of particles in a sunscreen on minipigs and found no significant penetration into intact normal epidermis (Toxicol. Sci. 2010;115(1):156-66).

In 2011, Furukawa et al. studied the postinitiation carcinogenic potential of coated and uncoated TiO₂ nanoparticles in a two-stage skin carcinogenesis model using 7-week-old CD1 (ICR) female mice. They found that application of coated and uncoated nanoparticles after initiation and promotion with 7,12-dimethylbenz[a]anthracene and 12-O-tetradecanoylphorbol 13-acetate at doses of up to 20 mg/mouse failed to augment nodule development. The investigators concluded that TiO₂ nanoparticles do not exhibit postinitiation potential for mouse skin carcinogenesis (Food Chem. Toxicol. 2011;49(4):744-9).

Human data

Given the persistent concerns about possible side effects of coated TiO₂ and ZnO nanoparticles used in physical sun blockers, Filipe et al., in 2009, assessed the localization and potential skin penetration of TiO₂ and ZnO nanoparticles dispersed in three sunscreen formulations, under realistic in vivo conditions in normal and altered skin. The investigators examined a test hydrophobic formulation containing coated 20-nm TiO₂ nanoparticles and two commercially available sunscreen formulations containing TiO₂ alone or in combination with ZnO, with respect to how consumers actually used sunscreens compared with the recommended standard condition for the sun protection factor test. They found that traces of the physical blockers could be detected only at the skin surface and uppermost area of the stratum corneum in normal human skin after a 2-hour exposure. After 48 hours of exposure, layers deeper than the stratum corneum contained no detectable TiO₂ or ZnO nanoparticles. While preferential deposition of the nanoparticles in the openings of pilosebaceous follicles was noted, no penetration into viable skin tissue was observed. The investigators concluded that significant penetration of TiO₂ or ZnO nanoparticles into keratinocytes is improbable (Skin Pharmacol. Physiol. 2009;22:266-75).

The weight of evidence

Current evidence suggests minimal risks to human health from the use of TiO₂ or ZnO nanoparticles at concentrations up to 25% in cosmetic preparations or sunscreens, according to Schilling et al., regardless of coatings or crystalline structure. In a safety review of these ingredients, they noted that these nanoparticles formulated in topical products occur as aggregates of primary particles 30-150 nm in size, and bond in such a way that renders them impervious to the force of product application. Thus their structure remains unaffected, and no primary particles are released. The authors also noted that nanoparticles exhibit equivalence with larger particles in terms of distribution and duration and, therefore, recognition and elimination from the body (Photochem. Photobiol. Sci. 2010;9:495-509).

But in 2011, Tran and Salmon, in light of findings that nanoparticles may penetrate the stratum corneum under certain conditions, considered the possible photocarcinogenic results of nanoparticle sunscreens. They noted, though, that most such results were obtained through the use of animal skin models, not investigations with human skin (Australas. J. Dermatol. 2011;52:1-6). To this point, the weight of evidence appears to show that such TiO₂ nanoparticles are safe when applied to intact human skin (Semin. Cutan. Med. Surg. 2011;30:210-13).

In response to the increased scrutiny and concern exhibited by the general public and government agencies regarding the safety of TiO₂ and ZnO nanoparticles, Newman et al. reviewed the literature and position statements from 1980 to 2008 to ascertain and describe the use, safety, and regulatory state of such ingredients in sunscreens. They found no evidence of significant penetration deeper than the stratum corneum of TiO₂ and ZnO nanoparticles, but caution that additional studies simulating real-world conditions (i.e., sunburned skin and under UV exposure) are necessary (J. Am. Acad. Dermatol. 2009;61:685-92).

Conclusion

Titanium dioxide is a well-established, safe, and effective physical sunblock. Nanotechnology has introduced some cause for concern regarding its use in physical sunblocks. In particular, evidence suggesting that photoexcitation of TiO₂ nanoparticles leads to the generation of reactive oxygen species that damage DNA, potentially launching a cascade of adverse events, has prompted investigations into the safety of TiO₂ in nanoparticle form. However, to date, multiple studies suggest that TiO₂ nanoparticles do not penetrate or are highly unlikely to penetrate beyond the stratum corneum.

Dr. Baumann is chief executive officer of the Baumann Cosmetic & Research Institute in Miami Beach. She founded the cosmetic dermatology center at the University of Miami in 1997. Dr. Baumann wrote the textbook "Cosmetic Dermatology: Principles and Practice" (McGraw-Hill, 2002), and a book for consumers, "The Skin Type Solution" (Bantam, 2006). Dr. Baumann has received funding for clinical grants from Allergan, Aveeno, Avon Products, Galderma, Mary Kay, Medicis Pharmaceuticals, Neutrogena, Philosophy, Stiefel, Topix Pharmaceuticals, and Unilever.