Occupational health guideline: if the threshold limit values (TLV) for NO2 and NO are 5 ppm and 25 ppm, prolonged exposure to NO2 is most commonly associated with which health effect in humans?

Introduction / Context:
Nitrogen dioxide (NO2) is a reactive gas produced by high-temperature combustion (vehicles, boilers, furnaces). Occupational and ambient standards limit chronic exposure because NO2 irritates the respiratory tract and exacerbates airway disease.

Given Data / Assumptions:

• TLV values: NO2 5 ppm, NO 25 ppm (illustrative limits).
• Exposure type: prolonged or chronic at elevated concentrations.
• Target organ system: respiratory system.

Concept / Approach:
NO2 is a strong oxidant that inflames the airway epithelium, promoting cough, reduced lung function, and susceptibility to infections. Chronic exposure links to bronchitic symptoms and aggravation of asthma/COPD. NO has lower irritancy at comparable levels.

Step-by-Step Solution:

Verification / Alternative check:
Epidemiological and chamber studies report reduced lung function and increased respiratory symptoms with chronic NO2 exposure, supporting the bronchitis association.

Why Other Options Are Wrong:

• Skin disorder/Acute dermatitis: primary effect is not dermatological.
• Bone disease: unrelated to NO2 pathophysiology.
• Cancer: NO2 is chiefly an irritant/oxidant; carcinogenicity is not the principal chronic endpoint in typical exposure ranges.

Common Pitfalls:
Confusing NO with NO2; assuming any pollutant primarily causes cancer; overlooking that oxidant gases chiefly target the respiratory tract first.

Final Answer:
Bronchitis