Although the etiology and risk factors are not completely understood, various genetic factors are responsible for AR. Family history of AR, sex, autoimmune diseases, human leukocyte antigen, and single nucleotide polymorphisms have been suggested as related factors through genetic or epidemiological studies. However, since it is currently difficult to modify these genetic factors, some researchers have focused on early-life environmental factors that could be amenable to intervention. Early-life environmental factors include pattern of delivery, breastfeeding, the number of siblings and living conditions, child’s infection history, including that of the mother during pregnancy, use of antibiotics, and other medications.

Numerous studies have investigated the association between the mode of delivery and allergic diseases, but have failed to reach a unanimous conclusion. However, the cesarean section seems to be associated with an increased risk of allergic diseases. Infants born by cesarean section lack contact with the bifidogenic bacteria in their mother’s vagina, which is thought to affect immune modulation and subsequent development of AR. Moreover, breastfeeding is associated with a decreased risk of allergic diseases, although studies on its influence have yielded conflicting results. A Swedish birth cohort study showed that exclusive breastfeeding in early infancy reduced the risk for eczema (odds ratio [OR], 0.78; 95% confidence interval [CI], 0.63 to 0.96) and onset of allergic march [3]. Another study in the United States showed that prolonged breastfeeding in African American subjects reduced the risk of AR (adjusted OR [aOR], 0.8; 95% CI, 0.6 to 0.9) at age 3 years [4]. The results from the Allergic Rhinitis Cohort Study for kids (ARCO-kids study) also showed that long-term breastfeeding is strongly associated with a decreased risk of AR in Korean children [5]. The possible protective mechanism conferred by breastfeeding is explained by its beneficial effects of lung development, as well as the presence of immune modulating factors in breast milk [6]. The number of siblings has been found to have protective effects in several earlier studies, including the previously mentioned Cincinnati study (aOR, 0.4; 95% CI, 0.2 to 0.8) [4]. The previous history of infection is a protective factor according to the hygiene hypothesis and our recent study showed that a previous history of pneumonia reduced the risk for AR (OR, 0.39; 95% CI, 0.19 to 0.82) [7]. A German study on bacterial infection in early life and the exposure of house dust mite (HDM) allergens in bed showed a contrasting effect of early endotoxin and HDM exposure for HDM sensitization [8]. HDM exposure increased the risk of mite sensitization at 5 years (aOR, 1.30; 95% CI, 1.11 to 1.53), while endotoxin exposure was inversely associated with HDM sensitization (aOR, 0.73; 95% CI, 0.57 to 0.95), which means bacterial exposure during early life may act as a protective factor against allergic diseases in childhood. However, another study with a birth cohort suggested that exposure to antibiotics is a potential risk factor in the development of allergic diseases [9]. In utero exposure to antibiotics was associated with an increased risk of asthma (adjusted hazard ratio [aHR], 1.68; 95% CI, 1.51 to 1.87), eczema (aHR, 1.17; 95% CI, 1.06 to 1.29) and AR (aHR, 1.56; 95% CI, 1.22 to 2.01). Antibiotic use in early life disrupts the infant gut microbiota and decreases their overall diversity, potentially leading to the development of allergic diseases. Therefore, bacterial exposure or the history of infection during early childhood should not be considered by itself, but in conjunction with the use of antibiotics to assess the exact effect on the development of AR. Interestingly, a recent cohort study found that acetaminophen, a widely used drug, increased the risk of AR by more than three times if used before the age of 3 [10].

Modification of genetic factors is difficult, but modification of the early-life environmental factors, including breastfeeding, mode of delivery, and living conditions, can prevent the development of allergic diseases including AR; it may also potentially reduce the increasing medical and socioeconomic burden associated with allergic diseases.