Which of the following can be considered a respiratory irritant?

Secondhand smoke is a well-known respiratory irritant due to the many harmful substances it contains, including tar, nicotine, and various chemicals that can provoke inflammatory responses in the lungs. Exposure to secondhand smoke can lead to various respiratory issues such as irritation of the airways, reduced lung function, and increased risk of respiratory infections. Additionally, it can exacerbate conditions like asthma and chronic obstructive pulmonary disease (COPD).

In contrast, fresh air is typically free of pollutants and serves to optimize respiratory function. Pure oxygen, although used in medical treatments, is not considered an irritant itself when administered properly; however, excessive concentrations can lead to toxicity. Hydrogen gas, while not toxic in normal environmental concentrations, does not carry the same harmful effects on the respiratory system as secondhand smoke does. Therefore, secondhand smoke stands out as a significant respiratory irritant among the options provided.