There are several strategies commonly employed to formulate sunscreens with broad-spectrum protection, preventing both short-term and long-term effects of UV radiation. Although it seems logical to combine various UV filters to achieve better UV protection, active ingredients (i.e., cosmetic ingredients) also play a significant role in optimizing the formulation for both efficacy and consumer acceptance. Additionally, developing and marketing new cosmetic ingredients is much easier than those UV filters, which also have to be approved by the FDA. Here is a summary of commonly used strategies;
 

• UVB protection is not enough. Therefore UVB filters should be combined with UVA filters to achieve optimal protection. Combining physical and chemical UV filters allows for better coverage of the UVB and UVA spectrums, resulting in broad-spectrum protection. Extending protection to the UVA region also helps enhance the SPF value.
   
• UV filters are either hydrophilic or lipophilic. When combined a synergetic effect can be observed. This property can be used to obtain the highest efficacy against UVB and UVA radiations. Additionally, as most sunscreen formulations are emulsions, having active ingredients in both phases provides better overall efficacy, even if the product is not uniformly distributed on the skin surface.
   
• The combination of organic and inorganic filters is also advantageous. Combining nano-TiO2 with chemical UV filters often provides better UVB protection than expected, based on the SPF of each ingredient. Studies show that as inorganic filters scatter light in the upper layers of the skin, they increase the optical path length of UV radiation and create more opportunities for absorption by chemical filters.
   
• Applying photo stabilizers is very important in all formulations containing chemical filters. As discussed earlier, many chemical filters, such as avobenzone, are sensitive to UV radiation. To prevent photodegradation, various photoprotective ingredients should be included in the formulations. Photostabilizers can quench the excited state of the organic filters by accepting the excited state energy, thereby returning the UV-absorbing molecule to its ground state.
   
• Homogenous distribution of the active ingredients in the product is also very important. Vehicles that dissolve and disperse the UV filters uniformly can enhance the overall UV protection by providing an even coverage on the skin, thus better protecting against sunburn. As inorganic filters are insoluble particles, they will be in a suspended state in the formulation. Rheology modifiers have to be used to provide an appropriate viscosity for the formulations. Ideally, products are easy to spread; however, they do not flow off the skin surface after application but stay there and form a uniform film.

Interesting Facts
Avobenzone is the only long-range chemical UVA filter (340-400 nm) widely available to sunscreen manufacturers in the US. However, the molecule is inherently unstable and by itself loses nearly 50% of its screening capacity after just 1 hour of UV exposure. Furthermore, the addition of other ingredients, even certain UVB filters to sunscreens, containing avobenzone accelerates the degradation of both compounds. This is why photo stabilizers are extremely important in formulations.

Formulation
The formulation of emulsions should follow the general formulation steps. Homogenous dispersion of the insoluble inorganic filters is very important for an effective and aesthetically appealing product. The formulation of gels should also follow the general steps, usually starting with hydrating the thickener(s). Sticks are formulated with the molding technique. 

The manufacture of aerosol formulations must be performed in a flameproof area using special equipment since they are highly flammable and/or explosive. First, the product concentrate is formulated, which is later filled into the can with the propellants using a special technique of pressure filling. In this process, the product concentrate is usually filled into the can first, and then the valve is inserted and crimped into place. Finally, the liquefied propellant is forced into the can under pressure through the valve orifice after the valve is sealed. Air can be removed from the can using a vacuum or displacement with a small amount of propellant vapor. After the filling operation is complete, the valve is tested for proper function.