South Africa may declare its biggest city and its capital as key areas to tackle air pollution, Patience Gwaze, the country’s national air quality officer, said.

Johannesburg, a city of about 5-million people, and Tshwane, the municipality that includes the capital, Pretoria, may be added to the list of priority areas, Gwaze said at a seminar on Monday.

About two decades ago, areas to the south and east of the cities, where the bulk of the country’s coal-fired power and petrochemical generation takes place, were declared priority areas to tackle some of the world’s worst air pollution. The declaration increases focus on air pollution monitoring and enforcement in these areas.