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Intranasal carbon dioxide (CO(2)) was shown to reduce symptoms of seasonal allergic rhinitis (SAR). This study was designed to evaluate the effect of CO(2) on nasal allergen challenge. We conducted a randomized, controlled, crossover trial in 12 subjects with SAR outside their pollen season. Thirty minutes after a 20-second exposure to CO(2) or no exposure, subjects underwent a unilateral, localized, nasal allergen challenge. Filter paper disks were placed on the nasal septum to deliver a sham challenge followed by 2 increasing doses of either grass or ragweed allergen. Secretions were collected from both sides of the septum to evaluate the nasonasal reflex and were assayed for histamine. Nasal and eye symptoms were recorded. The primary outcome measure was the contralateral, reflex, secretory response to allergen as measured by secretion weights. Secondary outcome measures included ipsilateral nasal secretion weights, nasal and eye symptoms, levels of histamine in nasal secretions, and eosinophils in nasal scrapings. Subjects reported a transient burning sensation during exposure to CO(2). Compared with no treatment, active treatment resulted in a significant reduction in sneezes (p = 0.05), contralateral secretion weights (p = 0.04), and bilateral runny nose symptoms (p = 0.01). Ipsilateral secretion weights were numerically reduced. Histamine levels in ipsilateral nasal secretions increased significantly when the subjects received sham treatment but did not increase after pretreatment with CO(2). Treatment with nasal CO(2) resulted in partial reduction of the acute response to allergen challenge. Reflex responses were reduced, supporting an effect on neuronal mechanisms, which predict usefulness in the treatment of allergic rhinitis. Registered with the U.S. National Institutes of Health clinicaltrials.gov. Identifier: NCT00618410.

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