Of all natural gas sources studied in the articles (S1), natural gas distribution pipelines were a prominent research target, and most studies used vehicle systems to measure emissions from pipeline leaks in city streets . U.S. east coast cities were emphasized geographically in the studies due to the higher prevalence of old, leak-prone cast-iron pipe in these cities. The number of leaks per kilometer of natural gas distribution pipeline in cities is closely related to material type; cities with more cast-iron pipe have leakier natural gas distribution pipeline networks than cities with low or no prevalence of cast-iron pipe .