The Bangkok Metropolitan Region, Thailand, has begun to experience air quality degradation, especially enhanced ozone (O₃) owing to elevated O₃ precursor emissions, strong solar radiation, high temperature and humidity. We present an observational analysis for 2010–2014, which reveals that the concentrations of O₃ exceeded the National Ambient Air Quality Standards of Thailand. The O₃ air quality index reveals evidence of standards being breached, resulting in potentially adverse health effects.