Transit operators move millions of people through enclosed spaces every day. In 2025, Americans took 8.1 billion trips on public transit, and cruise lines carried 37.2 million passengers.¹ ² For operators, the air inside those spaces must be effectively maintained and the filter in a vehicle or station air handler is where much of that work happens. The right filter depends less on a single efficiency rating than on how the system moves air, how much space the frame allows, how the filter is maintained and whether it fits the equipment already in place.
Air filtration needs vary widely across different forms of transportation, from commuter trains, subways, and buses to cruise ships.
Commuter Trains, Subways and Buses
Land transport carries the heaviest in-cabin particle load of the major transport modes. A review of 261 studies found that particles from traffic exhaust and from brake, wheel and rail wear enter cabins through doors and ventilation systems, and that a high-efficiency cabin filter reduced ultrafine particles and PM2.5 inside taxis.³ Underground systems add their own source, since braking and wheel-on-rail contact generate metal-rich dust in tunnels and on platforms.
For an operator, the first question is practical. Where does the air that reaches passengers come from, and does it pass through a system that can filter it? A filter only treats the air moving through its own unit. From there, the equipment sets the terms: the airflow the system moves, the filter depth the housing allows, the pressure drop the fan can carry, and how well the filter seals in its frame. Those limits, not the rating alone, decide what can be installed and kept in service, and they shape the maintenance schedule and how often a filter has to be changed.
Camfil’s AQ13, 30/30 Dual 9 and Hi-Flo ES are particle filters for building HVAC systems such as those in stations and terminals, and they also serve vehicle and vessel systems where the equipment suits them.
The Hi-Flo ES is a bag filter for stations and terminals that have the depth to house it. It runs from MERV 11 through MERV 15, carries a 12-month guarantee, and can be used without a prefilter to hold down energy use.
The AQ13 is a pleated panel filter rated MERV 13/11A that Camfil describes as fully compliant with ASHRAE 241. It comes in one, two and four-inch depths, which helps in transit equipment where frames are often shallow. Camfil positions its low pressure drop as a reason it can suit a move from MERV 8 to MERV 13. Whether that upgrade works in a given system depends on the system itself: the fan’s capacity, the space available and the pressure drop the equipment can accommodate. A higher-efficiency filter should not be assumed to drop in without checking those limits, and some systems will need adjustment to take it.
The 30/30 Dual 9 is a pleated panel filter rated MERV 9 and 9A with a dual-layer media in a moisture-resistant beverage board frame. Camfil guarantees it to remain in service for 12 months in commercial-duty applications and 9 months in industrial applications. It is used both as a prefilter ahead of higher-efficiency filters and as a stand-alone filter in rooftop units, package systems and air handlers. It captures a smaller share of fine particles than the AQ13, so the choice between the two comes down to the particles a system needs to control and what the equipment can carry.
Service life and operating cost are related but separate considerations. A filter that creates less airflow resistance can lower fan-energy consumption, and a longer-lasting filter can mean fewer changeouts and less labor, but neither is automatic. The gain depends on the system, the dust load and how the filter is maintained. Standard sizes help keep the right filter on hand. Custom sizing is available on the pleated AQ13 and 30/30 Dual 9, though not on the Hi-Flo ES or Durafil ES3.
Particle filters do not remove exhaust gases or the chemical vapors from cleaning products. Those call for source control, ventilation, or a carbon or molecular filtration stage built for gases and odors, used alongside the particle filter rather than in place of it. In vehicle passenger areas, that stage is Camfil’s CityPleat. For stations and terminals, it could be one of several products. The CityPleat is a combination particle and molecular filter in a pleated panel filter format and can be used wherever pleated filters are commonly used. The CityCarb I is also a combination particle and molecular filter in a V-bank configuration and often used wherever final filters are installed. Finally, for heavy-duty applications, the CamCarb XG is a cylinder filter containing molecular media and installed in special housings. This is a solution for extreme levels of gases and odors that need to be controlled.
Cruise Ships and Passenger Ferries
A ship has two filtration problems in one hull. Passenger spaces such as cabins, dining rooms and public areas run on HVAC systems much like a building’s. The mechanical areas, engine rooms and machinery spaces have their own ventilation and a dirtier, harder-working air supply. The filter choice differs between them. Large passenger and vehicle ferries share the same split, so the same filter choices apply.
Outdoor air at sea is not automatically clean. A survey of four cruise ships measured particle counts on open decks and found the highest readings behind the smokestacks. On one ship, the average near a running track behind the stacks was about 12,700 particles per cubic centimeter, against roughly 1,500 at the bow.⁴ Air drawn into a ship’s intakes carries part of that load, which is one reason intake filtration matters.
In passenger spaces, the same particle filters used on land apply where the HVAC system suits them. The AQ13 brings MERV 13/11A filtration where depth is limited, and the 30/30 Dual 9 serves as a prefilter or stand-alone panel. Mechanical areas place different demands on a filter: higher dust loads, continuous running and little tolerance for downtime. Two Camfil filters fit these spaces. The Hi-Flo ES is a bag filter available from MERV 11 through MERV 15, with a 12-month guarantee, and it runs with or without a prefilter. The Durafil ES3 is a V-bank filter available in MERV 13A, 14A and 16A. Both are used in ship mechanical areas.
As on land, odors and gases are a separate task. Galley exhaust, fuel and machinery produce odors that a particle filter does not touch, and a carbon or molecular stage is what addresses them. Since space is a premium on vessels, the CityPleat or CityCarb I are often ideal choices.
How Camfil Helps Transit Operators Get the Right Filter Fit
Even a well-chosen filter underperforms when it is fitted poorly. Camfil’s service procedure calls for checking frames and gaskets, fitting the filter in the direction of airflow, and recording the installation date. A good seal keeps air from bypassing the filter instead of passing through it, and the recorded date gives the maintenance team a reference for inspection and replacement. On systems that run continuously, that record is often what keeps a change from being missed until airflow drops.
For a transit operator, filter selection is an equipment decision before it is a product decision. The filter has to match the system’s airflow and the space in the housing, seal properly in its frame, fit a maintenance schedule the team can hold to, and be available in the sizes the fleet uses. Efficiency rating matters, but only within those limits. The practical next step is a system-by-system review with Camfil to match each filter to the application it is documented for, rather than choosing one filter for the whole operation.
Sources
- American Public Transportation Association, APTA Public Transportation Ridership Update, April 2026. https://bit.ly/47xxE8F
- Cruise Lines International Association, 2026 State of the Cruise Industry, April 2026. https://bit.ly/3WnyYsb
- Wang, Haimei, et al. A Global Multidimensional Analysis of Air Pollution in Land, Sea, and Air Transport Cabins Reveals Substantial Health Risks. One Earth, vol. 8, no. 1, 2025, article 101156. https://bit.ly/47y34M7
- Kennedy, Ryan David. An Investigation of Air Pollution on the Decks of 4 Cruise Ships. Stand.earth, 2019. https://bit.ly/4z6qDaO
- Camfil 30/30 Dual 9 Pleated Panel Air Filter Guarantee (Guarantee-Camfil-3030-Dual-9-Pleated-Panel-Air-Filter.pdf).
- Camfil Hi-Flo ES Guarantee Letter (Guarantee-Hi-Flo-ES-Guarantee-Letter.pdf).
- Camfil Durafil ES3 Guarantee (Guarantee-Durafil-ES3-Guarantee.pdf).
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