Air pollution exceedance events in Melbourne and Sydney, Australia, between 2000 and 2024

• Both cities comply with national air quality standards but fail to meet WHO guidelines. • Limited spatial and temporal data coverage in Melbourne constrains long-term air quality assessments. • Since 2000, both cities have experienced single-pollutant and multi-pollutant air pollution events. • Overall, Sydney recorded more air pollution events compared to Melbourne. • Multi-pollutant events are more frequent in Sydney, while single-pollutant events are more common in Melbourne • Most multi-pollutant events mostly involve a combination of NO 2 , PM 2.5 . Air pollution events pose significant challenges to public health in Melbourne and Sydney, Australia’s most populous cities. This study evaluates publicly available data to understand the frequency of air pollution exceedance events beyond World Health Organization (WHO) guidelines, as well as national and state standards. The air quality of both cities generally complies with national standards but consistently fails to meet WHO air quality guidelines. Since 2000, Sydney recorded single-pollutant events on 43% of monitored days and multi-pollutant events on 14% of monitored days. In Melbourne, single-pollutant events were recorded on 42% of monitored days, and multi-pollutant events on 8%. In Sydney, NO 2 exceeded WHO guidelines on 52% of monitored days, PM 2.5 on 13%, PM 10 on 4%, and O 3 on 6%. In Melbourne, NO 2 exceeded on 47% of monitored days, PM 2.5 on 4%, PM 10 on 6%, and O 3 on 2%. Evaluating long-term, city-scale air quality is challenging due to significant variations in spatial and temporal data coverage, especially in Melbourne. Many monitoring stations have limited temporal coverage and do not consistently monitor all key pollutants, meaning the true extent of air pollution is likely not fully captured. Since 2017, Melbourne has had only five active monitoring stations, compared to over 15 in Sydney. This study demonstrates the urgent need for expanded air pollution monitoring networks in Australia’s largest cities and highlights the need for ongoing research into the impacts of multi- as well as single-pollutant exceedance events.