Diesel exhaust is one of the most concentrated sources of harmful fine particulate matter in the U.S., and its impact isn’t spread evenly across the country. States with major freight corridors, ports, rail yards, and industrial activity tend to carry a disproportionate share of the burden, since diesel PM concentrates most heavily near the routes trucks, trains, and ships actually travel. Here’s a look at the states where that burden shows up most clearly, based on available air quality and freight-pollution data.
California
California consistently posts the worst overall air quality among U.S. states, driven largely by industrial emissions and geography. Much of that burden traces directly to diesel sources. The state’s Central Valley is a particular hotspot — Bakersfield has ranked as the metro area with the worst year-round particle pollution in the country for seven consecutive years, a problem tied to a mix of farm dust, oil and gas activity, and diesel traffic moving along major freight routes like State Route 99 and Interstate 5, all compounded by valley terrain that traps pollution in place. The Los Angeles basin adds its own diesel burden on top of that: diesel exhaust accounts for more than two-thirds of the overall cancer risk from air pollution there, according to regional air quality regulators, with roughly 90% of trucks serving the Ports of Los Angeles and Long Beach still running on diesel fuel.
Texas
Texas’s major freight and industrial corridors have pushed it further up recent pollution rankings. The Dallas-Fort Worth metro area moved from 30th to 22nd worst for year-round particle pollution in the most recent national ranking, one of several metro areas newly appearing on the worst-25 list. The state’s dense highway freight network, oil and gas activity, and port traffic all contribute diesel emissions on top of its broader industrial footprint.
Ohio
Ohio has been flagged alongside California as a state facing significant air quality challenges tied specifically to industrial emissions. Its position along major Midwest freight and rail corridors, combined with heavy industry and winter temperature inversions that trap pollution near the ground, compounds the diesel-related share of its particulate pollution problem.
Pennsylvania
Pennsylvania’s exposure is closely tied to its role as a major East Coast freight and logistics hub. The Philadelphia-Reading-Camden metro area, spanning Pennsylvania, New Jersey, Delaware, and Maryland, moved from 26th to 17th worst for year-round particle pollution in the most recent national assessment. Heavy truck traffic connecting to major East Coast ports, combined with regional industrial activity, keeps diesel particulate levels elevated across the corridor.
Missouri
Missouri’s inclusion reflects its position as a major inland freight and shipping crossroads. The St. Louis–St. Charles–Farmington metro area, spanning Missouri and Illinois, continues to carry a failing grade for particle pollution even after some recent improvement. The region’s role as a rail, river shipping, and highway freight hub — where diesel-powered trucks, trains, and barges all converge — keeps diesel particulate exposure persistently high for nearby communities.
Why These States Stand Out
A common thread runs through all five: heavy concentrations of diesel-powered freight movement, whether by truck, train, or ship, layered on top of existing industrial activity. Diesel particulate matter is especially harmful because the highest levels are found near ports, rail yards, and freeways — the exact places where trucks, buses, trains, and ships full of diesel engines cluster, and where nearby residents breathe the exhaust directly. People living and working near these corridors, rather than the general state population, tend to carry the heaviest share of diesel-related health risk, even in states with better statewide averages.
Join The Discussion
Do you live near a major freight corridor, port, or rail yard, and have you noticed air quality concerns tied to diesel truck or train traffic in your area? Share what local efforts — cleaner truck fleets, port electrification, buffer zones — you’ve seen make a difference, or questions you have about how diesel pollution is measured and regulated where you live. If you’re researching this topic for a specific state or community, feel free to ask questions — pooling local knowledge here can help paint a fuller picture than any single ranking can.
