I’m working on my senior thesis looking at the effects of air pollution health and income disparity. I’m focusing on doing this at the microlevel using the PUMA variable in IPUMS USA because it’s the smallest geographic variable I could find. However, the NHIS data is only for regions, is there a way to write a code in stata that will allow me to merge the NHIS and USA data that way each reflects observations at the puma level?

As you note, the lowest geographic level identifiable in the NHIS public use microdata is the region. This limitation prevents any sort of identification of which lower level geographic areas (e.g., states, counties, or PUMAs) the observations reside within these regions. Therefore, there is no way to merge IPUMS USA and IPUMS NHIS data at the PUMA level.