Studies in a number of cities (Bangkok, Cairo, Jakarta,
Santiago and Tehran, to name five) have assigned priority
to controlling lead and particulate matter
concentrations, which present the greatest hazard to
human health. Where photochemical ozone is a problem
(as it is, for instance, in Mexico City, Santiago, and
São Paulo), control of ozone precursors (nitrogen oxides
and volatile organic compounds) is also important
both because of the damaging effects of ozone itself and
because of the secondary particulate matter formation
resulting from atmospheric reactions with ozone. Carbon
monoxide and toxic air contaminants have been assigned
lower priority for control at the present time,
but measures to reduce volatile organic compounds exhaust
emissions will generally reduce carbon monoxide
and toxic substances as well.