17.—(1) The Secretary of State must ensure that levels of sulphur dioxide, nitrogen dioxide, benzene, carbon monoxide, lead and particulate matter do not exceed the limit values set out in Schedule 2.
(2) In zones where levels of the pollutants mentioned in paragraph (1) are below the limit values set out in Schedule 2, the Secretary of State must ensure that levels are maintained below those limit values and must endeavour to maintain the best ambient air quality compatible with sustainable development.