Allergies are a major health issue, the sixth-leading cause of chronic illness in the United States, with an annual cost more than $18 billion. More than 50 million Americans suffer from allergies, such as bronchial asthma, allergic rhinitis and atopic dermatitis, each year.

Some allergy sufferers report that symptoms like sneezing, sniffling, itchy eyes and skin make them miserable, impacting their daily lives and activities. And they may not be exaggerating.

Some experts explain this allergy-depression connection in psychological terms of the heavy emotional toll of chronic allergy symptoms.

Historically, there is not a direct link between the mechanism of allergic disorders and depression or anxiety. Early cross-sectional studies reported associations between allergies and major depression, but in the absence of longitudinal data, the implications of the association were unclear.

A later study supported the idea that major depression was associated with an increased risk of developing non-food allergies, but there was no confirmation of an effect in the opposite direction. Even with research hinting at causal connections between allergies and emotional or psychological disorders, the overall picture has been unclear.

However, a new, large-scale study surprisingly links eczema, hay fever and asthma to an increased risk of developing mental illnesses. An analysis of nearly 200,000 people showed a link between allergic rhinitis, asthma and atopic dermatitis with psychiatric disease.

Dr Nian-Sheng Tzengfrom Tri-Service General Hospital in Taiwan and researchers used the database of the Taiwan National Health Insurance Program. A total of 186,588 enrolled patients, with 46,647 study participants who had suffered from allergic diseases, and 139,941 controls matched for sex and age, from the Longitudinal Health Insurance Dataset of 2000-2015, were selected from a sub-dataset of the National Health Insurance Research Database.

The researchers used Fine and Gray's competing risk model analysis to explore the hazard ratio and 95 percent confidence interval for the risk of allergic diseases being associated with the risk of developing psychiatric disorders during the 15 years of follow-up.

Dr. Tzeng's suspicions that those with eczema, asthma and hay fever had more emotional, conduct and hyperactivity problems were confirmed. The groups of atopic dermatitis alone, and the allergic rhinitis plus atopic dermatitis were associated with a lower risk of psychiatric disorders, but the other four groups, such as bronchial asthma alone, allergic rhinitis alone, bronchial asthma plus allergic rhinitis, and bronchial asthma plus atopic dermatitis, and the combination of all three allergic diseases, were associated with a higher risk of psychiatric disorders.

Across the duration of the study, 6.7 percent of participants without allergic conditions developed a psychiatric condition. For those with an allergic condition, this figure rose to 10.8 percent, a 66 percent increase.

While this study cannot demonstrate why the relationship between allergic and psychiatric conditions exist, the researchers hope to alert clinicians to assess and monitor the mental health of their allergic patients.

Looking forward, novel methods to treat mental health issues may result from this evidence of the link between psychiatric health and inflammation.