The modern world is highly industrialized and urbanized. One accompaniment of this change in lifestyle is the rise of food allergies. A new research paper explores associations between food allergies and pre- or post-natal exposure to pollutants in the air, thus paving the way for future studies on the potential sensitization of the gut by exposure of the skin or airway to pollutants, in addition to the alimentary route.
Study: Early-life exposure to air pollution associated with food allergy in children: Implications for ‘one allergy’ concept. Image Credit: Africa Studio / Shutterstock
Food allergy is defined as "a specific immune response to particular foods" and is estimated to affect about one in ten people worldwide. This proportion is probably still higher in children. In some cases, the exposure may bring on a life-threatening anaphylactic reaction to the concerned food, often occurring within seconds or minutes. Food allergies are responsible for many emergency medical visits, a great deal of medical expenditure, and a restriction of the opportunity to participate in social gatherings involving eating. This could also lead to social isolation, bullying, and low mood. Thus, food allergies pose a health risk, an emotional challenge, and a financial burden on the patient and caregiver, as well as on society, due to the accompanying ill health and loss of productivity.
Food allergies are different from other allergies that make up the "atopic march," namely, asthma, allergic rhinitis, and eczema, which represent a progression of disease in childhood. One notable divergence is the 30-year-delay in the rise in the prevalence of food allergies compared to the first wave of allergies that included the other three conditions, leading to its being dubbed the "second wave" of allergy.
Secondly, food allergies are due to gut exposure to allergens, while the others are due to skin and airway exposure. However, scientists are currently reconsidering this hypothesis in light of evidence that skin exposure can also lead to sensitization to food allergens, called the "gut-skin dual exposure hypothesis.
Air pollution plays a significant role in the evolution of other allergies, but less is known about its part in food allergies. The current paper published in the journal Environmental Research aims to identify associations, if any, between air pollution exposure and the recent rapid increase in the incidence of food allergies in China. This would support their triple exposure hypothesis of food sensitization involving gut, skin, and airway exposure.
"If this hypothesis is true, it is believed that air pollution has led to the first and second waves of allergy epidemics, which indicates a concept of 'one allergy' disease."
The study was carried out on a cohort of children who participated in the "China-Child-Family-Health (CCHH)" project between September 2011 and January 2012. The researchers asked about children's food allergies, indoor surroundings, and lifestyle. Over 2,500 children from 36 kindergartens (3-6 years of age) took part, and their parents answered the questionnaires.
Food allergy prevalence over the child's lifetime was asked using the International Study of Asthma and Allergies in Childhood (ISAAC) form, based on the appearance of eczema, hives, swelling of the lips or eyes, or diarrhea following the consumption of a specific food.
Outdoor air pollution was measured in the form of three pollutants, namely, sulfur dioxide (SO2), nitrogen dioxide (NO2), and particulate matter with a diameter ≤10μm (PM10), being markers of industrial pollution, traffic pollution, and mixed pollution respectively. The average concentration of pollutants per day was used to estimate the daily exposure for each child, depending on the child's distance from the measuring station.
In addition, indoor air pollution was represented by the presence of new furniture, redecoration, mold or dampness, and condensation on windows. The latter two represent the adequacy of ventilation, and the first two are significant sources of airborne pollutants.
Prenatal exposure was defined as being within the period of pregnancy, by trimester, and post-natal from the first post-natal month to the last month before the questionnaire was administered.
The researchers excluded demographic variables that could confound the results and other factors like whether the household had dogs, smokers, and the cleaning habits used in general.
What did the study show?
About one in seven children were reported to have a food allergy, with an increased risk in males, those who had parents with atopic conditions, and if the house was rarely cleaned. The risk was highest in the age group of 3-4 years compared to those aged 5-6.
The risk of food allergies was increased in children who had prenatal exposure, as when their parents opted for house redecoration and new furniture during pregnancy, as well as if the house was poorly ventilated. For example, when mold/dampness was reported during pregnancy, the offspring were twice at risk for food allergies. Conversely, when the baby breathed in fumes from new furniture or lived in a poorly ventilated home, food allergies increased by 50% and 40%, respectively.
Outdoor air pollution was also linked to food allergies in childhood. For example, NO2, reflecting air pollution due to heavy traffic, increased the risk of food allergies by 25% and 38% between quartiles of exposure, depending on whether the exposure occurred pre- or post-natally. Similarly, PM10 and SO2 were linked to ~40% and 30% increases in the risk of food allergies during childhood following post-natal exposure.
Earlier studies showed an increase in egg or milk allergies in children exposed to volatile organic compounds (VOCs) relative to unexposed children. Similarly, propylene glycol and glycol ethers (PGEs) are commonly emitted from new furniture. Exposure to these substances in the bedroom increased the risk that the child would show signs of hypersensitivity by 80%.
Overall, air pollution appears to be associated with childhood FA between three and four years of age. This corroborates the findings of earlier studies, showing, for instance, an increase in peanut allergy at one year of age with higher NO2 levels.
What are the implications?
The study provides some evidence that food allergies can arise through airway exposure, supporting the one-allergy school of thought.
"Our studies suggest that both the first wave of allergy epidemic (including eczema, asthma and allergic rhinitis) and the second wave of allergy epidemic are sensitive to the same environmental exposures, especially air pollutants."
This could help control this second wave of food allergies that is sweeping the developed world since the ability to control one form of allergy, such as asthma, by purifying the inhaled air, for instance, could be valuable in the prevention of food allergies as well.
"Our study indicates that the rapid increase of childhood food allergies in China is linked with the development during the past decades."
A perhaps skewed pattern of economic development led to a massive exodus from the countryside into the cities, coupled with a vast increase in air pollution from a surge of industrial development.
Scientists have reported that babies as young as two years in Chongqing had food allergies in almost 8% of cases, vs. only 3.5% in 1999. This rising trend is also visible in other industrialized countries like Canada, the USA, and the UK.
"It can be expected that food allergies will further increase in the near future, particularly in low- and middle-income countries" (LMIC) because these nations measure their welfare regarding economic growth and urbanization. The findings of this study could indicate new methods of intervention to prevent or treat such allergies since all arise from the same root cause."
Further research should use a prospective design including more air pollutants to correctly estimate the prevalence of food allergies and rule out an association with the reverse directionality of cause-effect.