Abstract

Atopic diseases have increased in prevalence over the last few decades and the rapid increases suggest that the predominant driving forces behind these increases are environmental factors rather than genetic alterations. A number of environmental factors have been implicated in the increased prevalence of allergic diseases. Predominant among them are increased exposure to pollutants and decreased exposure to microbes and parasitic infections. The hygiene hypothesis suggests that increased hygiene and lack of exposure to microbes and parasitic infections at an early age prevents the necessary stimulus to train the developing immune system to develop tolerogenic responses. Lifestyle factors, such as increased time spent indoors, use of antibiotics, and consumption of processed foods and decreased exposure to farm animals and pets, limit exposure to environmental allergens, infectious parasitic worms, and microbes. The lack of exposure to these factors is thought to prevent proper education and training of the immune system. Other factors that are also associated with increased risk of allergic diseases are Caesarian birth, birth order, tobacco smoke exposure and psychosomatic factors. Here, we review current knowledge on the environmental factors that have been shown to affect the development of allergic diseases and the recent developments in the field.