¿Qué controles pasa un fotoprotector? | Por qué nunca deberías realizar crema solar casera

Si mezclamos 2 cucharadas de óxido de zinc con 5 cucharadas de aceite de sésamo, aloe vera y vitamina D obtenemos un fotoprotector solar con un SPF de 30. Así de simple.

¿Por qué firmas cosméticas invierten tanto dinero en crear nuevas fórmulas cuando existen recetas caseras tan sencillas?

Hay tantas variaciones de cremas solares caseras, como recetas de gazpacho. Aceite de frambuesa o de coco, dos o tres cucharadas dependiendo del SPF que queramos alcanzar o unas gotitas de aceites esenciales (potencialmente alérgenos y fotosensibilizantes) para que huela un poco mejor.

Lo cierto es, que aunque realmente esta mezcla tuviese un SPF30, nunca lo sabremos. Ya que nunca se someten a ningún control de calidad ni se mide su SPF real.

¿Cómo se testa un protector solar? | Regulación Europea

Cada zona geográfica tiene su propio reglamento. De ahí que muchas empresas puedan comercializar fotoprotectores solares en Europa pero no en USA y viceversa.

En Europa las cremas solares están reguladas según el reglamento (CE) 1223/2009. Dentro del estándar existe el método COLIPA (de la European Cosmetics Trade Association) que determina qué ensayos deben realizarse para obtener el SPF (obligatorio en todas las cremas solares) y UVA (solo si queremos usar la etiqueta UVA). Se combinan estudios in vivo e in vitro para poder obtener una mejor reproducibilidad y, además, tener en cuenta la posible biodegradación de la fórmula cosmética.

Los ensayos o test que se realizan son tres:

Protección UVA

  • Ensayos in vivo: se conoce como ensayo de oscurecimiento pigmentario permanente (PPD) e inmediato (PPI).  Los sujetos del estudio (personas) se exponen a una fuente controlada de UVA y se mide el tiempo hasta que aparece oscurecimiento de la piel.
  • Ensayos in vitro: se mide la capacidad de una muestra de transmitancia frente a la radiación UV.  Con este ensayo se obtiene el factor de protección UVA: UVAPF y UVAPF0 (antes de la exposición).

En Europa no es necesario usar la nomenclatura PPD para hacer referencia a la protección UVA. Si no que aparece el símbolo UVA en un círculo para indicar que como mínimo el UVAPF es 1/3 del factor SPF. En cambio, en Japón si se usa esta nomenclatura, etiquetado bajo las siglas PA en función del valor del PPD.

Optar por usar esta nomenclatura en nuestras cremas solares no estaría nada mal. Pero viendo el jaleo que ya tenemos con los filtros químicos y físicos, se convertiría en un auténtica locura.

Factor de protección solar (SPF)

Como ya hablamos aquí, el SPF nos da el valor que relaciona cuánto tiempo se tarda en que aparezca eritema mínimo (MED) con la piel protegida y sin proteger. Que suele estar relacionada con la radiación UVB.

  • Ensayos in vivo: se realiza en sujetos con fototipos bajos (I, II-II) y cuya última exposición solar haya sido superior a dos meses. En la zona de la espalda se aplica el equivalente a 2 mg/ cm2 y se expone de manera controlada a una fuente de radiación ultravioleta después de unos 15-30 minutos de la aplicación.Se irradian tres zonas: blanco (zona sin proteger), muestra (fotoprotector a medir) y referencia, y se calcula el tiempo que tarda en aparecer eritema en la piel (cuando los bordes se empiezan a enrojecer).
  • Ensayos in vitro: al igual que los ensayos de UVA están relacionados con la transmitancia.

Se aconseja realizar ambos test para evitar sesgos y realizar análisis comparativos, aunque los que son obligatorios son los ensayos in vivo.

Como detalle decir que las personas que realizan la parte de preparación de las muestras (como sería extender la crema), no son los mismos que evalúan posteriormente el SPF, para evitar así una interpretación errónea y sesgada.

¿El SPF indica cuanto tiempo puedo estar expuesto al sol sin quemarme?

No. Las condiciones de un laboratorio distan mucho de las condiciones reales.

Las radiaciones solares inciden a lo largo del día de manera variable.  Existen picos de índice de radiación ultravioleta (UVI) muy variables, lo que hace que tengamos un UVI a las 10:00 am y otro muy distinto a las 15:00 pm.

Artículo recomendado: Índice Ultravioleta y SPF

Longitud de onda crítica

La definición exacta de este método es: longitud de onda a la cual el área bajo la curva del espectro de absorbancia para el producto irradiado es del 90% del total. En el caso del reglamento europea esta no debe ser inferior a 370 nm.

Otros 

Como habrás podido observar, existen fórmulas water resistant o waterproof. Ambos términos sólo pueden usarse cuando los test diseñados para comprobar si la fórmula es resistente al agua han sido aprobados.

El proceso para poder determinar el SPF de un fotoprotector solar es bastante costoso y largo. Todas las cremas que podemos encontrar en el mercado europeo deben cumplirla, tanto las de supermercado como las de farmacia.

Qué ocurre en una crema solar casera

Aquí repasamos por qué no debemos usar un aceite vegetal como protector solar. Aceites vegetales como el de oliva no superan un valor de SPF de 7. En el caso del aceite de sésamo, ampliamente usado en cosmética casera, el SPF no llega a 2.

Dejando de lado que no podemos saber con exactitud el SPF de una crema solar, este tipo de recetas presenta un pequeño problemilla: sistema de medida y homogeneidad de la muestra.

Una cucharadita de café o un chupito de aceite no es un sistema de medida universal. ¿Cuántos gramos contienen una cucharadita rasa? ¿Con copete? Pues eso, precisión y exactitud nula.

Y el otro gran problema: no son dispersiones homogéneas.

La ardua tarea de dispersar un óxido metálico

Los óxidos metálicos son complicados de mezclar ya que son prácticamente insolubles en todo. Por eso, cuando hablamos de fórmulas solares con filtros minerales siempre se menciona que es una dispersión, nunca una solución. Los filtros físicos NO se disuelven, se dispersan. Lo que realmente tenemos es una suspensión, donde las partículas están suspendidas o dispersadas.

Para ayudar a que la dispersión/suspensión cosmética sea mucho más homogénea, se pueden usar algunas técnicas o métodos. Los más habituales actualmente son:

  • Usando micro- o nanopartículas. Tienen un tamaño mucho más pequeño y facilitan su dispersión. Además, son mucho más cosméticas y evitan dejar ese rastro blanquecino en la piel cuando lo aplicamos
  • Membranas, liposomas o polímeros como recubrimiento de los óxidos metálicos. Ayudan a dispersar mejor las partículas, ya que suelen tener afinidad por la fase oleosa (orgánica)
  • A parte de esto, muchas fórmulas contienen emulsionantes. Compuestos químicos que ayudan a que la fase oleosa y acuosa se «mezclen».

Por eso, cuando realizamos una crema casera y la dejamos reposar se separan las fases: el pigmento se deposita en el fondo (lo que se conoce como sedimentación), y el aceite arriba.

Al no ser homogénea, cuando aplicamos el producto en la piel tendremos zonas donde pongamos más pigmento, y zonas en las que menos. De manera que no estaremos aplicando la cantidad suficiente de manera uniforme.

Además de esto, tenemos otro problema: no conocemos el tamaño de partícula, relacionado con la protección UVA y UVB. Es decir, no sabemos con exactitud la protección ultravioleta que nos está ofreciendo.

Y tú, ¿te animas a realizar tu protector solar en casa?

Bibliografía

[1] Standarisation CEN mandate assigned to concerning methods for testing efficacy of sunscreen products, European Comission.

[2] RECOMENDACIÓN DE LA COMISIÓN de 22 de septiembre de 2006 relativa a la eficacia de los productos de protección solar y a las declaraciones sobre los mismos, Diario Oficial de la Unión Europea.

[3] Cosmetics Europe: N 16, Water resistance labelling, 2005.

153 pensamientos en “¿Qué controles pasa un fotoprotector? | Por qué nunca deberías realizar crema solar casera

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