Air pollution levels in the cities of Bogotá and Medellín (Colombia) are now reaching dangerous levels, as measured by the few government monitoring stations available and operative. So far, government actions have focused on enforcing private car and industrial emission limits, prohibiting the circulation of vehicles for a few days during particularly polluted periods and vague promises of switching a percentage of public transport vehicles to alternative power. Official measurements are increasingly being reported in the national and local media, as well as through social media, but citizen distrust of their quality has mounted. Citizen collectives are starting to work designing low-cost mobile sensors, monitoring pollution in some areas and in public transport, sharing their georeferenced data over the internet and trying to raise awareness of the dangers of pollution and the necessity of radical actions to deal with the problem. This paper describes the current actions of some of these citizen collectives and their results in setting the media and public agenda on air pollution problems.