Filtros solares: físicos y químicos | Conceptos básicos

En cuanto llega Julio y Agosto se disparan las ventas de protectores solares y con ello los titulares alarmistas sobre su toxicidad. Es tal la cantidad de información disponible que para el consumidor se ha convertido en un dolor de cabeza.

Hace unos días en una conversación sobre cremas solares infantiles me dí cuenta de que la información llega mal en España, en Alemania y en todas partes. Todos coincidían en que no debían contener nanopartículas, pero ninguno se aclaraba si también eran malos los metales (filtros físicos), o si lo que era perjudicial eran los químicos. Al final, la mayoría estaba usando justo lo que los dermatólogos no aconsejan en bebés menores de un año, filtros químicos. Eso sí, ninguna contenía nanopartículas.

Tipos de filtros en el mercado

  • Filtros inorgánicos (minerales), conocidos como filtros físicos. Los usados y permitidos actualmente: óxido de titanio y óxido de zinc.
  • Filtros orgánicos, conocidos como filtros químicos. Los más comunes: avobenzone (muy inestable y difícil de formular), Tinosorb, Meroxyl, octocrylene, homosalate, etc.
  • Filtros biológicos, es decir, antioxidantes. Su función es neutralizar los radicales libres. Y realmente no son filtros en sí, sino antioxidantes.

Radiación solar – Espectro electromagnético

Espectro-electromagnetico-uva1-uva1

Básicamente tenemos tres tipos de radiación solar: ultravioleta, infrarroja y la luz visible. Cada una cae en una longitud de onda diferente del espectro electromagnético:

  • Ultravioleta: UVA (400 – 320 nm), UVB (320 – 280 nm) y UVC (280- 200 nm).
  • Luz visible: (400 – 800 nm)
  • Infrarrojo: IRA (800 – 1200 nm)

Post recomendado: UVA, UVB, UVC, IR-A, Visible ¿Cuál es más peligrosa?

Filtros físicos o filtros minerales

Los que actualmente están permitidos en la EU son el dióxido de titanio (TiO2) y el óxido de zinc (ZnO). Crean una película sobre la superficie de la piel que actúa como filtro frente a la radiación ultravioleta.

Son insolubles en la fórmula cosmética y nunca estarán disueltos si no que están dispersos en el medio. La ventaja que tienen: que son fotoestables, no se degradan con la luz.

Cuando la radiación ultravioleta incide sobre los óxidos metálicos absorben energía ultravioleta con una longitud de onda entre 400-280 nm (UVA+UVB). Al absorber esta energía los electrones de la capa basal se excitan y pasan a un estado excitado en la banda de conducción. Posteriormente, vuelven a su estado normal y como consecuencia emiten una radiación con una energía menor que la incidente.

La energía emitida tiene una determinada longitud de onda, en el caso de los filtros físicos corresponde a una longitud dentro del espectro visible,y por eso, vemos el color blanco.

absorcion-emision-radiacion-solar-filtro-fisico-quimico
Modelo de excitación y emisión de radiación genérico. En el caso de los óxidos metálicos tendríamos la capa de conducción y capa basal.

Nanopartículas de óxidos metálicos

De forma sencilla, cuando usamos un filtro mineral al uso hablamos de un material que tiene un diámetro de partícula alrededor de los 200 nm, y que no consideramos como material nanoparticulado (bulk material). En cambio, cuando hablamos de nanopartículas nos referimos a partículas cuyo diámetro está entre  los 100-0 nm, que se obtienen tras procesos físico-químicos.

¿Qué aportan las nanopartículas? Aunque hablamos extensamente aquí, de manera resumida ofrecen:

  • Mejor formulación: al usar un material nanoparticulado el proceso de dispersión se vuelve más sencillo y permite que las dispersiones sean más homogéneas.
  • Aumenta la eficacia de protección frente a la radiación solar: debido a un aumento el área superficial.
  • Son más estéticos, ya que eliminan el efecto blanquecino en la piel.

¿Penetran en la piel? La gran mayoría de las nano-partículas se quedan depositadas en el estrato córneo (capa más externa de la epidermis), y a no ser que tengamos una herida abierta, son inocuas. De todas formas, la unión europea solo permite el uso de nano-partículas con un tamaño superior a 30 nm y además deben estar recubiertas de alúmina o sílice cuando se combinan con filtros químicos para evitar su aglomeración, posible reacción con otros filtros, etc, lo que aumentaría su tamaño final.

filtros solares

Post recomendado: Filtros físicos: nanopartículas en cremas solares ¿tóxicas?

Filtros químicos u orgánicos

Como hemos visto antes, los filtros minerales emiten en el visible y por tanto se “ven blancos” un problema que se soluciona en parte gracias a la nanopartículas.  Para solucionar este efecto blanco tan anti estético «nacieron» los filtros químicos, aunque su nombre real debería ser filtros orgánicos, ya que son moléculas a base carbono, oxigeno e hidrógeno.

Se consideran filtros químicos los compuestos que poseen un grupo cromóforo en su estructura, que son capaces de absorber la radiación ultravioleta. Los cromóforos son grupos aromáticos o grupos con enlaces dobles conjugados, que contienen electrones capaces de “saltar”.

Su funcionamiento es similar a los filtros físicos, solo que en este caso, la energía procedente de la radiación ultravioleta se transforma en calor (radiación térmica).

La ventaja de los filtros químicos es la ventana tan amplia que existe de protección UVA y UVB. Permitiendo una combinación perfecta entre ellos o con filtros físicos, y por último, son más específicos, ya que algunos de ellos no protegen en todo el rango de UVA, si no solo a una porción: UVA-1 (400 – 340 nm) o UVA-2 (340 – 320 nm), siendo la UVA-1 la más dañina, y con mayor poder de penetración (dermis).

filtros-solares-químicos

Sensibilidad química

El problema de los filtros orgánicos es su incompatibilidad con pieles sensibles o reactivas. Al ser orgánicos suelen ser bastante afines a la piel y pueden penetrar (que no atravesar) superficialmente en el estrato córneo, generando posibles irritaciones o dermatitis de contacto.

Como en todo, existen pieles que pueden toleran mejor o peor este tipo de compuestos, pero productos destinados a pieles sensibles están testados dermatológicamente para que no ocasionen reacción cutánea.

En el caso de bebés y niños menores de tres años la recomendación oficial es usar filtros minerales para evitar dermatitis por contacto. Al igual que una piel muy reactiva.

¿Cúal es mejor?

Ambos. La combinación de los dos filtros solares nos cubre tanto el UVA como el UVB. Y si además añades algún antioxidante extra, mejor.

 


Bibliografía

[1] Smijs, Threes G, and Stanislav Pavel. “Titanium Dioxide and Zinc Oxide Nanoparticles in Sunscreens: Focus on Their Safety and Effectiveness.” Nanotechnology, Science and Applications 4 (2011): 95–112. PMC. Web. 25 June 2018.

[2] Seyhan Eye, Química orgánica: estrucutura y reactividad, Volumen 2, Editorial Reverté, 2000.

[3] T. Maier H.C. Korting, Sunscreens- Which and What for? Skin Pharmacol Physiol 2005;18:253–262

[4] L.K. Dennis, L.E. Beane Freeman, M.J. Van Beek, Sunscreen use and the risk for melanoma: a quantitative review. Ann Intern Med 2003;139:966-78.

[5] Filtros solares. Características, tipos y requerimientos, Deiry Marína, Alfonso del Pozoa, Formación permanente en dermofarmacia, Elsevier, 2005.

[6] Fotoprotección infantil , Leire Azcona, Farmacéutica comunitaria de Bizkaia. Máster en Cosmética y Dermofarmacia, 2005.

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