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The “five times worse” claim refers to ultrafine particles measured inside diesel-hauled train carriages compared with a busy street in Copenhagen—not to every pollutant, every train, or overall health risk. A separate Danish study found higher levels of all assessed pollutants inside diesel-train cars than inside electric-train cars, but the size of the difference varied by pollutant.
What does “five times worse” mean?
Chemistry World reported ultrafine-particle concentrations above 200,000 particles per cubic centimetre in diesel-hauled Copenhagen train carriages, compared with an average of 40,000 particles per cubic centimetre on a busy Copenhagen street. That is the source of the approximate five-to-one comparison. It concerns one pollutant and one specific outdoor reference setting; it is not a general score for train air quality or a direct measure of health risk. The trains in that report dated from the 1980s. Chemistry World’s 2019 report.
How did diesel-train air compare with electric-train air?
A 2019 study by Maria Helena G. Andersen and colleagues measured personal exposure inside passenger cars on diesel and electric trains in the Copenhagen area. Monitoring took place for 6–7 hours per day over 49 days on diesel trains and 22 days on electric trains. The study found higher average concentrations of every assessed pollutant in diesel-train passenger cars. The ratios below are diesel-versus-electric comparisons from that study—not comparisons with street air.
| Pollutant | Diesel-train concentration | Increase versus electric-train cars |
|---|---|---|
| Ultrafine particles | 212,000 particles/cm³ | 35-fold |
| Black carbon | 8.3 μg/m³ | 6-fold |
| NOx | 316 μg/m³ | 8-fold |
| NO₂ | 38 μg/m³ | 3-fold |
| PM₂.₅ | 34 μg/m³ | 2-fold |
| Benzo[a]pyrene | 0.14 ng/m³ | 6-fold |
These figures are average concentrations reported for the study conditions. They should not be read as typical values for every diesel train or as a current survey of all fleets. The study found that concentrations varied with pull or push mode, whether an engine was operating, and the passenger car’s distance from the locomotive. See the study, “Exposure to Air Pollution inside Electric and Diesel-Powered Passenger Trains”.
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How does the separate sidewalk comparison differ?
A 2017 study of diesel-powered commuter trains compared conditions in pull-mode train cars with city sidewalks. It reported ultrafine particles at five times, lung-deposited surface area at three times, and black carbon at four times the levels measured while walking on sidewalks. Exposure was greatest in the coach closest to the locomotive. These are pollutant-specific ratios from a different study and outdoor comparison; they are not the same measurements as the Copenhagen busy-street figure or the 2019 diesel-versus-electric study. The 2017 commuter-train study.
Why can exposure differ from one carriage to another?
Train configuration and operating conditions matter. The 2019 Copenhagen measurements varied with pull versus push operation, engine use, and distance from the locomotive; the 2017 study likewise found the greatest exposure in the coach nearest the locomotive. Chemistry World quoted University of Toronto researcher Greg Evans recommending that passengers sit as far from the engine as possible when a train is being pulled. That is a practical suggestion based on reported findings, not a guarantee that any particular seat is safe or that every train has the same exposure pattern.
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Ultrafine particles are a particular focus in the report: study lead Maria Helena G. Andersen told Chemistry World that they are believed to travel deeper into the lungs. The presence of higher measured concentrations, however, does not by itself quantify the health effect of an individual journey.
What do the health findings establish?
A separate controlled study exposed 29 healthy volunteers to diesel-train exhaust for six hours per day on three consecutive days, with electric trains as a control condition. It reported reduced lung function and increased DNA strand breaks in peripheral blood mononuclear cells; several other measured outcomes were unchanged. These short-term results do not establish long-term disease risk for ordinary passengers, nor do they show the effect of a single trip. The volunteer exposure study in Particle and Fibre Toxicology.
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How to interpret the comparisons
- Check the pollutant: the headline comparison is about ultrafine particles, while the diesel-versus-electric study measured several pollutants with different ratios.
- Check the reference: a busy Copenhagen street, city sidewalks, and electric-train passenger cars are distinct comparators.
- Check the operating conditions: pull or push mode and engine use affected measured concentrations.
- Check the setting: the findings come from defined Danish train conditions, including older diesel stock; they do not establish levels on every current rail fleet.
- Separate concentration from risk: measured air pollution and short-term volunteer responses do not quantify an individual passenger’s long-term health risk.
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