During the first two decades of the twenty-first century, we analyze the expansion of urban land cover, urban heat island (UHI), and urban pollution island (UPI) in the Houston Metropolitan Area (HMA) using land cover classifications derived from Landsat and land/aerosol products from NASA's Moderate Resolution Imaging Spectroradiometer. Our approach involves both direct utilization and fusion with in situ observations for a comprehensive characterization. We also examined how social vulnerability within the HMA changed during the study period and whether the synergy of UHI, UPI, and social vulnerability enhances environmental inequalities. We found that urban land cover within the HMA increased by 1,345.09 km2 and is accompanied by a 171.92 (73.93) % expansion of the daytime (nighttime) UHI. While the UPI experienced an overall reduction in particulate pollution, the magnitude of change is smaller compared to the surroundings. Further, the UPI showed localized enhancement in particulate pollution caused by increases in vehicular traffic. Our analysis found that the social vulnerability of the HMA urban regions increased during the study period. Overall, we found that the urban growth during the first two decades of the twenty-first century resulted in a synergy of UHI, UPI, and social vulnerability, causing an increase in environmental inequalities within the HMA. We used a combination of satellite and ground-based observations to study how urban area expansion in Houston, Texas affects urban heat, pollution, and social vulnerability. We found that in the past two decades, Houston has added to its urban land cover by almost twice the area of New York City. This resulted in warmer conditions during the daytime which persisted during the night. Although there was an overall decrease in particulate pollution, our research identified pockets of heightened traffic activity that offset these improvements. We also found social vulnerability increased during the last two decades. We find the combined effect of increased heat stress, pollution and vulnerability leads to increased environmental inequalities. The rapid urbanization of Houston expands the urban heat and pollution islands, raising exposure to high temperatures and poor air quality Communities with highest social vulnerability are most impacted by the adverse effects of urban heat and pollution islands The Clean Air Act's efficacy is locally diminished by Houston's urban pollution island
