Cold temperatures and low humidity can also increase susceptibility to mechanical stress and immunological activity. Keratinocytes release pro-inflammatory cytokines and cortisol, and dermal mast cell numbers increase, resulting in the skin being more reactive to allergens and skin irritants.19

Seasonal allergens may therefore influence skin conditions. Atopic eczema and pruritis can increase a few days after birch pollen peaks, being worse on skin exposed to the air rather than covered up. Birch pollen can penetrate the skin via hair follicles, but may also have a surface allergen effect through the stratum corneum damaged by proteases. Pollen-associated lipid mediators can then lead to a Th2 (T-helper cell) response, enhancing the inflammation.20

The birch pollen season starts in March, although other common tree pollens (hazel, yew, elm, alder, willow and ash) can peak as early as the end of February.21

Pollens may also cause cross sensitisation to other allergens, such as in foods. Atopic dermatitis has been reported to worsen with symptoms appearing 6-48 hours after ingesting birch cross-reactive food allergens.22,23,24