Longtail boats: romance at the expense of your health? Exhaust, pollutants and the environment

These points are backed by the cited sources. Everything else in the article is context.

The longtail boat gets its name from the long drive shaft with the propeller at its end. The engine stands open at the stern or amidships, often without any cover. In Thailand converted engines from cars and trucks, diesel or petrol, are often used for this. Such an engine is not built to run close to people, and as a rule it has no exhaust after-treatment such as the particulate filter or catalytic converter of modern road vehicles.

The pictures in this article show longtail boats with a dark plume of smoke and an engine with a turbocharger. They do not come from tours in Krabi, and these are evidently heavily tuned boats. They therefore do not show a typical tour boat. They do show how visible soot can become when a diesel engine runs under load.

The exhaust of a combustion engine consists of gases and particles. The gases include carbon monoxide, nitrogen oxides and volatile organic compounds. The particles include soot (black carbon), fine particles (PM2.5) and ultrafine particles with a diameter below 100 nanometres. Further substances cling to the particles, among them polycyclic aromatic hydrocarbons (PAHs). How much of it is produced depends on the engine, fuel, maintenance and load.

In a 2012 report the WHO describes black carbon as a component of fine particles that forms during incomplete combustion. According to its review, studies show an association between daily variations in black carbon concentration and short-term health effects, and between long-term exposure and mortality from cardiovascular and lung disease. Toxicology suggests that soot may act less as a toxic agent itself and more as a carrier for other chemicals into the lungs and blood.

In June 2012 the IARC, the WHO's cancer agency, classified diesel engine exhaust as "carcinogenic to humans" (Group 1). The basis was sufficient evidence of a link to lung cancer, plus limited evidence for bladder cancer. The IARC rates petrol exhaust lower, as "possibly carcinogenic" (Group 2B). A trigger for the re-evaluation was, among others, a US study of underground miners. A 2018 review reports that re-analyses of two large studies confirmed the original findings.

Important for putting this in context: the classification says that a substance can cause cancer, not how large the risk is at a given dose. The studies concerned people with years of exposure at work. A single boat trip is not comparable. For skippers and crew who stand next to the engine every day, the picture is different from that of a guest who rides once.

According to a review of their physicochemical properties, ultrafine particles are so small that they can reach the finest regions of the lung (the alveoli) and bypass the airways' first lines of defence. The same work notes that harmful effects of PM10 or PM2.5 are often attributed to the ultrafine fraction. In its 2021 air quality guidelines the WHO lowered the annual guideline for PM2.5 from 10 to 5 micrograms per cubic metre and highlighted black carbon and ultrafine particles as important topics.

Soot and particles irritate the airways and can trigger coughing, shortness of breath or asthma attacks in sensitive people. Risk groups include children, pregnant women, older people and people with asthma, COPD or heart disease. This is general medical knowledge about air pollutants, not a measurement on a boat in Krabi.

For longtail boats in Krabi specifically we found no measurements of passengers or crew. That is a gap. The nearest source is a project by the Climate and Clean Air Coalition (CCAC) with the Asian Institute of Technology on boats on the canals and the Chao Phraya in Bangkok. It found that boats with 15 to 20 year old engines emit a lot of black smoke when leaving and approaching piers and contribute to Bangkok's air pollution. The emissions of this sector had not been studied separately before.

In model calculations of the same project, switching to engines meeting the Euro VI standard with low-sulphur fuel would cut PM2.5 emissions by about 98 percent, while cleaner fuel alone with the existing engines would cut them by only about 5 percent. This concerns Bangkok's canal boats and cannot be transferred one to one to Krabi. It does show that above all the engine decides, not the fuel. The project's participants also note that the overall effect may be small, but that people living along the water and the boat operators are likely to be affected most.

Put honestly: a single trip on a longtail boat does not, on today's knowledge, lead to cancer in healthy adults. The risk arises from dose and duration, and duration mainly concerns the crew. For guests, complaints in the moment are possible: smell of exhaust, an irritated throat, headaches, shortness of breath in sensitive people. There is also the noise of an open engine, which makes a trip tiring even without exhaust.

If you are sensitive, or travel with small children, pregnant women or people with breathing or heart conditions, you should keep these points in mind. And if you are attached to a romantic moment, you can have the romance another way: quieter, without a plume of smoke, at a distance from strangers' groups.

We do not run a boat with a converted vehicle engine. Our private speedboat runs on two modern Suzuki outboard engines (300 hp each, 4.0-litre V6 four-strokes), developed for use on boats. According to the manufacturer, the Lean Burn technology of these engines adjusts the air-fuel mixture via an oxygen sensor in the exhaust to reduce consumption and emissions. On top of that, the boat is yours alone for the day, together with up to 5 guests: no strangers, no changing boats, no waiting for others. You decide departure time and pace, and we adapt seating and route if you are sensitive to exhaust or travel with children.

The price is per boat for up to 5 guests. Some islands and national parks charge an entrance fee that is not included in the tour price and is paid on site directly to the rangers. The start is possible in Ao Nang or Krabi Town.

Exhaust and noise are not only a matter for people. Consequences of boat traffic are documented in Thailand's marine parks. A park official explained in 2018 that over about 20 years boats mooring in Maya Bay had damaged reefs and disturbed the sand. The bay was therefore closed to boats. According to a 2024 report, on average about 120 boats, including longtails and speedboats, enter Pileh Bay every day. Air pollution from exhaust, noise and a higher risk of accidents are cited. The authorities therefore banned empty longtail boats from parking in the lagoon and from transferring guests from speedboats.

Mooring on the reef is also considered damage. From Koh Kradan it was reported that a boat was tied to coral although a buoy was available, which led to legal action against its owner. Corals grow slowly: some species need decades or longer. This is not specific to longtails but a risk of any motorboat without buoys and rules.

On noise: studies show that motorboat noise can affect the behaviour and reproduction of some reef fish. In a Great Barrier Reef study, nesting male fish spent more time chasing other fish and less time guarding their young when boat noise was played. A 2021 review describes noise as a growing stressor on coral reefs but notes that most studies cover only a few fish species and that the effect depends strongly on species, life stage and type of noise. For corals themselves we found no direct measurements.

On fuel and oil that reach the water from longtail boats, we found no published measurements. In general, spilled fuel and oil harm marine life. Ask every operator how it handles refuelling, oil changes and bilge water.

Especially for families, pregnant women and people with breathing or heart conditions, a short check is worthwhile before you book a boat.

The health statements rest on these sources. Where they state limits, we have kept them in the text.