Air pollution is a silent development crisis. It is the leading global environmental risk factor for early death and a major driver of preventable illness, undermining economic productivity and sustainable development. The purpose of this policy brief is to deepen the understanding on why ambient air quality (AQ) investment remains constrained in low- and middle-income countries, despite the growing political recognition of the harms. It is based on findings from a four-month study involving desk reviews and key informant interviews with multilateral development banks and government stakeholders across 15 countries. The brief discusses key barriers to public air quality financing and offers recommendations to governments and international finance institutions to unlock and scale up funding.