An Inflammatory Question? Prenatal Air Pollution, Childhood Allergic Rhinitis, and Healthy Fats

Short abstract

A study of more than 650 toddlers found that two polyunsaturated fatty acids were associated with fewer cases of allergic rhinitis (hay fever) in children who had been exposed prenatally to higher levels of PM_2.5.

Exposure to ambient fine particulate matter (${\mathrm{PM}}_{2.5}$) during pregnancy poses health concerns for both the mother and fetus. Prenatal exposure has been associated with an increased risk of developing allergic conditions, such as childhood asthma, wheezing, food allergy, atopic dermatitis, and allergic rhinitis, or hay fever.^1,2 A rising prevalence of childhood allergic rhinitis^3–5 and an earlier age at symptom onset⁶ have been reported in China, where urban ${\mathrm{PM}}_{2.5}$ levels are typically higher than in Europe and North America.⁷ There is evidence that polyunsaturated fatty acids (PUFAs)—nutrients in seafood and certain nuts, seeds, and plant-based oils⁸ —may help alleviate the harmful effects of ${\mathrm{PM}}_{2.5}$.^9,10 A prospective birth cohort study recently published in Environmental Health Perspectives¹¹ further explored the role of PUFAs in the relationship between prenatal ${\mathrm{PM}}_{2.5}$ exposure and allergic rhinitis in the city of Guangzhou, China.

The present study found a lower association between prenatal exposure to particulate matter and childhood hay fever when certain healthy fats were consumed during pregnancy. To reduce exposure to polluted air, people can take protective measures that include limiting time outdoors, wearing N95 face masks, and using indoor air filters.³⁵ Image: © iStock.com/VioletaStoimenova.

The new study involved 657 mother–child pairs and included maternal blood samples and child health outcomes up to 2 years of age. The researchers found that each increase of $5{\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$ in prenatal ${\mathrm{PM}}_{2.5}$ exposure was associated with a 1.9-fold increased risk of the child being diagnosed with allergic rhinitis, as well as a 1.8-fold increase in parent-reported symptoms, which included nasal congestion, sneezing, and watery, itchy red eyes.¹² These hazard ratios reflect adjustment for potential confounding factors such as maternal smoking and age, family socioeconomic status, and residential greenness.

To test whether any of six PUFAs modified associations between ${\mathrm{PM}}_{2.5}$ and allergic rhinitis, the researchers measured levels of these fats in maternal red blood cell membranes, at 20–28 weeks of gestation. They found that each increase of $5{\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$ in prenatal ${\mathrm{PM}}_{2.5}$ exposure was associated with an adjusted 3.7-fold increased risk of allergic rhinitis symptoms in the children of mothers with high levels of linoleic acid (LA), and a 3.4-fold increased risk of symptoms was associated with low maternal levels of docosapentaenoic acid (DPA). In contrast, prenatal ${\mathrm{PM}}_{2.5}$ exposure was not associated with symptoms in children of mothers with LA levels below or DPA levels above the sample median.

Allergic rhinitis can be challenging to diagnose at 2 years of age,¹³ because it is difficult for children this young to recognize and communicate symptom triggers. However, it may continue into adolescence and adulthood¹⁴ and may increase the risk of developing other allergic diseases.¹⁵ For example, in one study the presence of allergic rhinitis in 2-year-old Japanese children was associated with a 2-fold greater risk of developing asthma 2 years later.¹⁶

The new study used a Chinese air pollution dataset¹⁷ to estimate prenatal ${\mathrm{PM}}_{2.5}$ exposure as the maternal exposure averaged over the pregnancy period, using the geocoded residential address. These concentrations (mean $35.5{\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$) were similar to other reports for Asia ($33–72{\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$)^18–20 and higher than those observed in Europe and North America ($8–17{\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$).^21,22

Because ${\mathrm{PM}}_{2.5}$ is a mixture of substances, the authors also estimated model-based¹⁷ average daily exposures to five major constituents to assess their relative contributions to the joint effect. These five constituents were sulfate, nitrate, ammonium, organic matter, and black carbon. Using a statistical method for mixture analysis, the researchers showed that, of the five constituents, black carbon made the largest contribution to the association between increased ${\mathrm{PM}}_{2.5}$ exposure and risk of allergic rhinitis and its symptoms.

“Traffic emissions are the most important source of black carbon in urban Guangzhou,²³ followed by coal combustion, soil dust, and ship emissions,” says Li Cai, the paper’s senior author and an associate professor of maternal and child health at the city’s Sun Yat-sen University. “Animal models and in vitro experiments support the toxicity of black carbon,”²⁴ she says; however, the relative contribution of different ${\mathrm{PM}}_{2.5}$ constituents likely depends on a given region’s distinct mix of emission sources. Cai notes that the reduction of traffic emissions is especially important for Guangzhou.

PUFAs—chains of lipids with at least two chemical double bonds—are considered healthy fats.²⁵ For example, fish oil is taken as a dietary supplement because it is high in PUFAs known as omega-3 fatty acids.²⁶ Dietary recommendations for pregnant women include PUFA-rich foods owing to their health benefits for the growing fetus.²⁷ Because PUFAs modulate inflammatory and immune processes, their ability to reduce the harmful effect of air pollution is biologically plausible.^28,29

The new study suggests that high maternal DPA and low maternal LA levels may potentially protect children against the ${\mathrm{PM}}_{2.5}\text{-associated}$ risk of allergic rhinitis. The DPA result, says Li, was surprising because the researchers were expecting to find such effects with eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)—two other omega-3 PUFAs with well-known favorable health effects; however, other studies are increasingly recognizing DPA as an omega-3 with benefits for early-life development.³⁰

Emily Oken, a professor of population medicine at Harvard Medical School who was not involved in the study, notes that atopic dermatitis and food allergy are more commonly diagnosed than allergic rhinitis in North American toddlers.³¹ That is also true in Europe, according to Ulrike Gehring, an associate professor of environmental child health at Utrecht University in the Netherlands and an associate editor for Environmental Health Perspectives.

“Allergic rhinitis is typically diagnosed in school-age children in northern Europe³² and is rare until then,” says Gehring, who also was not involved in the study. “Measuring allergen-specific immunoglobulin E levels in serum [to assess the severity of allergic rhinitis³³] and following the cohort over time would be very interesting, to see if the observed association persists and if the children develop other allergic conditions later in life.”

Oken says the findings are another reason to recommend the regular consumption of fish or other sources of PUFAs during pregnancy. “We already know that a good fatty-acid status supports several desirable pregnancy outcomes, such as a reduced risk of preterm birth and better cognitive health for the baby,^25,34” she says. “Adding a lower risk of allergic rhinitis to the list of benefits is important, especially in high-pollution settings with limited opportunities to reduce ${\mathrm{PM}}_{2.5}$ exposure.”

Li points out the need to strike a balance between the benefits of consuming seafood and the potential risks of contaminant exposure during pregnancy. Pregnant women should remain well-informed about their options, such as choosing low-mercury species, she says, and adhere to public health recommendations regarding servings and local fish advisories. Ultimately, Gehring adds, “Although nutrition can be a way to reduce some of the air pollution effects, improving air quality should remain the priority.”

Biography

Silke Schmidt, PhD, writes about science, health, and the environment from Madison, Wisconsin.