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In one Copenhagen comparison, ultrafine-particle levels inside diesel-hauled train carriages were about five times the average measured beside a busy street. That figure applies to ultrafine particles in a specific setting—not to every pollutant, every train, or overall health risk. A separate Danish study found higher average concentrations of all the pollutants it assessed inside diesel-train cars than inside electric-train cars, with the size of the difference varying by pollutant.
What the five-times comparison means
Chemistry World reported ultrafine-particle concentrations above 200,000 particles per cubic centimetre in diesel-hauled carriages, compared with an average of 40,000 particles per cubic centimetre beside a busy Copenhagen street. The rounded comparison is therefore about five to one. It is a comparison of ultrafine-particle counts in those reported settings, not a general rating of train air versus street air. Chemistry World’s 2019 report noted that the Copenhagen trains studied dated from the 1980s.
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Ultrafine particles are one pollutant measure; the number does not say that every pollutant was five times higher, nor does it quantify a passenger’s total exposure or resulting health risk. Lead author Maria Helena G. Andersen told Chemistry World that ultrafine particles are of concern because they are believed to travel deeper into the lungs.
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A 2019 study by Andersen and colleagues measured pollution inside passenger cars of diesel and electric trains in the Copenhagen area. Personal exposure monitoring took place for 6–7 hours per day over 49 days on diesel trains and 22 days on electric trains. The study found significantly higher average concentrations of every assessed pollutant in diesel-train cars. Its reported diesel-versus-electric comparisons were:
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| Pollutant | Diesel-train average | Difference from electric-train average |
|---|---|---|
| Ultrafine particles | 212,000 particles/cm³ | 35-fold higher |
| Black carbon | 8.3 μg/m³ | 6-fold higher |
| NOx | 316 μg/m³ | 8-fold higher |
| NO₂ | 38 μg/m³ | 3-fold higher |
| PM₂.₅ | 34 μg/m³ | 2-fold higher |
| Benzo[a]pyrene | 0.14 ng/m³ | 6-fold higher |
These are pollutant-specific diesel-versus-electric average concentration comparisons from the study, not the busy-street ratios. Concentrations varied with whether a train was in pull or push mode, which engine was operating, and how far the passenger car was from the locomotive. The study, published in Environmental Science & Technology, describes its methods and results.
Why train position and operating mode matter
Train air measurements are not uniform across every car or operating condition. In the Copenhagen work, levels differed with pull or push mode, engine use, and distance from the locomotive. A separate 2017 study of diesel-powered commuter trains also found the greatest exposure in the coach nearest the locomotive.
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That 2017 study compared pull-mode train measurements with readings taken while walking on city sidewalks. It reported ultrafine particles at five times sidewalk levels, lung-deposited surface area at three times, and black carbon at four times. These are separate pollutant-specific comparisons and should not be combined with either the Copenhagen busy-street figure or the 2019 diesel-versus-electric results. The 2017 commuter-train study provides that sidewalk comparison.
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In 2019, University of Toronto researcher Greg Evans advised passengers to sit as far as possible from the engine when a train is being pulled. This is practical advice based on the reported exposure patterns, not a guarantee that a seat is safe or that the same difference applies to every train.
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What the health evidence does—and does not—show
A separate controlled study exposed 29 healthy volunteers to diesel-train exhaust for six hours per day on three consecutive days, using electric trains as a control condition. The researchers reported reduced lung function and increased DNA strand breaks in peripheral blood mononuclear cells; several other measured outcomes were unchanged. The volunteer study examined short-term effects under a defined exposure protocol. It does not establish long-term disease risk for ordinary passengers or show what a single trip does.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How broadly to apply the findings
The headline comparison comes from reported measurements in Copenhagen, and the diesel/electric findings come from specific Danish train settings. The Copenhagen report describes trains from the 1980s. These results show that diesel-train passenger-car air can have substantially higher measured pollutant concentrations in the studied conditions; they do not establish present-day levels for every diesel fleet, route, or carriage. The relevant comparison depends on which pollutant was measured, the outdoor or train reference setting, operating mode, engine use, carriage position, and study design.
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