The stability of pyrethrins in formulations containing different combinations of antioxidant and sunscreen was studied with model system experiments. The mechanism of disappearance of pyrethrins was mainly due to photodegradation, but some loss by codistillation can occur. In formulations, the presence of mineral oil and emulsifiers determined a decrease on codistillation losses. The presence of the antioxidant in formulations even at high concentrations did not affect pyrethrin photodegradation rates, but rising amounts of sunscreen determined a progressive increase on half-life times of pyrethrins. A combination of sunscreen and antioxidant at low concentrations provided an effective protection against sunlight similar to that obtained by high amounts of sunscreen alone.
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