The roadside air pollution index is expected to be very high in the next few days, the Environmental Protection Department warns. The index at Central and Causeway Bay roadside stations yesterday reached highs of 143 and 106 respectively. A department spokesman said nitrogen dioxide was the main pollutant. Intense sunshine and high temperature, influenced by the subsiding air of tropical storm Lionrock, had increased smog and ozone formation during the daytime and resulted in high background pollution, he added.