Delhi faces some of the world’s highest concentrations of PM2.5, the most damaging form of air pollution. Although awareness of outdoor air pollution is rising across the world, there is limited information on indoor air pollution (IAP) levels, particularly in heavily polluted cities like Delhi. Even less evidence exists on how IAP varies by socio-economic status (SES), and whether or not addressing information gaps can change defensive investments against IAP. In this paper, we deploy Indoor Air Quality Monitors (IAQMs) in thousands of Delhi households across varying socio-economic strata in order to document IAP levels during the peak wintertime air pollution period. Across high and low SES households, we document indoor PM2.5 levels that are: (1) extraordinarily high — more than 20 times World Health Organization (WHO) standards; (2) only 10 percent lower in high (versus low) SES households; and (3) significantly higher than levels reported by the nearest, outdoor government monitors, the main source of public information on air pollution in this setting. We then report on a field experiment that randomly assigned IAQMs, as well as an opportunity to rent an air purifier at a subsidized price, across medium and high SES homes during the 2019-20 winter season.