Perhaps the most difficult of the pollutants to control is:

Ozone is indeed one of the most challenging pollutants to control due to its nature and the complexity of its formation and impact. Ozone is not directly emitted into the air; instead, it forms in the atmosphere through chemical reactions between precursor pollutants such as nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. This secondary pollutant's formation is influenced by weather conditions, making it variable and difficult to manage.

Moreover, ozone is a significant component of smog and poses serious health risks, including respiratory problems and cardiovascular issues. It can also harm vegetation and ecosystems, leading to broader environmental impacts. The difficulty in controlling ozone levels is compounded by the need for coordinated efforts among various sectors, including transportation and industrial activities, to reduce the precursor pollutants.

In contrast, other pollutants like sulfur dioxide and nitrogen oxides have more direct sources and are often easier to target through specific regulatory measures. While carbon dioxide is a critical greenhouse gas, it is managed through long-term climate strategies, but its direct impact on air quality is less immediate compared to ozone. Therefore, the multifaceted nature of ozone pollution, combined with its health and environmental effects, underscores why it is particularly challenging to control.