Photochemical smog is a complex outdoor air-pollution mixture that most strongly affects the eyes, airways, and lungs. Health impacts vary with pollutant concentration, exposure duration, activity level, and individual susceptibility.

What about photochemical smog makes it harmful?

Photochemical smog contains strong oxidants and irritants that contact sensitive tissues (eyes and respiratory tract) and trigger inflammation.

Key health-relevant components commonly present in photochemical smog include:

• Ground-level ozone (O₃): the primary driver of many acute respiratory symptoms.

• Irritating organic compounds (e.g., aldehydes) that can sting eyes and airways.

• Secondary particles (fine particulate matter) that can penetrate deep into the lungs.

• Co-occurring urban pollutants that can worsen symptoms in combination (even when not the main “smog” indicator).

Exposure routes and why they matter

• Inhalation is the main route; higher breathing rates during exercise increase pollutant dose.

• Eye contact with oxidants and irritant gases contributes to the classic “smog sting.”

• Short, high exposures can cause symptoms even in healthy people; repeated exposures increase risk of longer-term impacts.

Immediate (acute) health impacts

The AP syllabus emphasises that photochemical smog irritates eyes and causes respiratory problems.

These are typically most noticeable on warm, sunny afternoons when oxidant levels are elevated.

Eye irritation

• Burning, stinging, and watering eyes

• Redness and increased sensitivity to light

• Contact lens discomfort due to tear-film disruption

Respiratory irritation and breathing problems

• Throat irritation and coughing

• Chest tightness, wheezing, and shortness of breath

• Reduced lung function during or after outdoor activity (you may feel “out of breath” sooner)

This figure shows how ozone exposure shifts the lung’s flow–volume relationship by reducing maximal inspiration, which lowers forced vital capacity (FVC) and related expiratory measurements. Visually connecting ozone to spirometry helps explain why people may feel chest tightness and reduced exercise capacity during high-smog afternoons. Source

• Asthma exacerbations (more frequent symptoms, increased rescue-inhaler use, more urgent care visits)

• Increased airway reactivity: the bronchi can narrow more easily in response to exercise or allergens

System-wide effects tied to airway inflammation

Even when symptoms feel “mild,” oxidative irritation can trigger inflammation that:

• increases susceptibility to respiratory infections

• worsens existing allergic rhinitis or seasonal allergy symptoms

• contributes to missed school/work days and reduced athletic performance

Longer-term (chronic) health impacts and vulnerability

Health risk rises with repeated exposure over time, especially in people with underlying cardiopulmonary disease.

Potential chronic outcomes

• Persistent airway inflammation that can contribute to chronic bronchitis-like symptoms

• Slower lung-function growth in children in high-smog regions (population-level effect)

• Greater risk of hospitalisation for respiratory disease during extended smog episodes

• Cardiopulmonary stress: fine particles and oxidants can increase strain on the heart and lungs, elevating risk for people with heart disease

Populations at higher risk

• People with asthma, COPD, or other chronic lung conditions

• Children and teenagers (higher outdoor activity and developing lungs)

• Older adults, especially with heart or lung disease

• Outdoor workers and athletes (higher ventilation rates and longer time outdoors)

Recognising and reducing personal health risk (without changing emissions)

Health protection focuses on lowering exposure during high-smog periods:

• Follow local air-quality alerts; limit prolonged outdoor exertion when oxidant levels are high

• Shift exercise to mornings/evenings when conditions are safer

• Seek medical advice if smog exposure repeatedly triggers asthma attacks or persistent cough