Nasal irritation and irritant-induced reflexes (rhinorrhea and congestion) are prominent symptoms associated with indoor and ambient air pollution, and marked heterogeneity in individual sensitivity has been suggested. Nevertheless, there is currently no generally accepted functional index of nasal irritant sensitivity available for clinical use. To address this issue, we compared two objective measures of nasal irritant sensitivity: a CO2 detection task, and CO2-induced transient disruption of respiratory pattern (pulsed CO2 acting as an odorless irritant). Using a respiratory flow thermocouple to produce a continuous recording of respiratory pattern, we challenged 20 normal adult volunteers (13 males and 7 females, average age 39.4 years) with brief (approximately 3 second) pulses of the odorless irritant carbon dioxide. Increasing levels of CO2 (10-70%, vol/vol), paired with filtered air in random order, were presented unilaterally by nasal cannula of fixed geometry, synchronized with the inspiratory phase of the respiratory cycle. All subjects yielded CO2 detection thresholds, whereas within the constraints of the testing method (subjective irritation rating < or = "very strong"), only 13 of 20 subjects (65%) exhibited transient disruption of their breathing pattern. Further, although decreased respiratory volume (indirectly measured) appeared to be a common feature, several distinct patterns of respiratory alteration were observed, rendering objective scoring more difficult. Finally, some subjects showed CO2-induced respiratory disruption intermittently from trial to trial, implying that rapid adaptation occurs. Determination of the CO2 detection threshold therefore appears to be the more objective and consistently applicable endpoint for determining individual nasal irritant sensitivity.