Nitric oxide (NO) plays a vital role in the physiology of the respiratory system, acting as a biomarker of airway inflammation. Two key noninvasive measurements—fractional exhaled nitric oxide (FeNO) and nasal nitric oxide (nNO)—are increasingly used to evaluate respiratory and nasal pathology. NO is primarily produced by inducible nitric oxide synthase (iNOS), which is expressed in various cells, including epithelial cells, macrophages, neutrophils, endothelial cells, and vascular smooth muscle cells [1]. The majority of the NO of the lung originates from the central airways, with minimal contribution from the alveolar or peripheral regions. While nNO serves as a marker for upper airway inflammation and is notably reduced in disorders like primary ciliary dyskinesia and increased in allergic rhinitis.

The existing reference values of FeNo and nNO have largely been derived from non-Indian populations, primarily Caucasian cohorts. Ethnic, environmental, and anthropometric differences significantly influence NO levels, and therefore, applying reference standards from other populations may not yield accurate clinical interpretations in the Indian context. Moreover, variations in measurement techniques and devices, such as chemiluminescence versus electrochemical sensors, further impact the generalizability of these values.

This study aims to fill this gap by determining the reference ranges and exploring the demographic and physiological predictors of FeNO and nNO levels among healthy Indian adults using standardized measurement techniques.