Gaseous air pollutants in the on-board environment

Where does it occur on board?

In the environment, nitrogen oxides (NOx) - nitrogen monoxide (NO) and nitrogen dioxide (NO2) - are formed by combustion at high temperatures. The main source of nitrogen oxides in the indoor environment is exhaust gases from internal combustion engines. Nitrogen dioxide is also produced during gas welding because the flame is directed directly into the air. Exposure to nitrogen dioxide caused several deaths in Swedish shipyards in the 1940s after gas welding in enclosed spaces.

In Sweden, emissions of nitrogen oxides to outdoor air have been more than halved between 1990 and 2020 as a result of reductions in emissions from industry and private transport, mainly due to catalytic cleaning of the exhaust gases.

Health effects

Nitrogen oxides are corrosive gases with low water solubility. Any exposure to NOx can be harmful to health, depending on the concentration, duration and route of exposure. Inhalation of nitrogen oxides can cause poisoning and skin exposure can cause corrosive damage. Lung and respiratory effects include dilation of the blood vessels in the lungs, which can lead to more severe asthma attacks in asthmatics. Similarly, severe chest pain and acute shortness of breath can be symptoms of exposure to nitrogen oxides. The low water solubility of nitrogen oxides means that they can penetrate into the alveoli of the lungs without the high levels of exposure being noticed. It may take several hours for symptoms of pulmonary oedema to appear, leading to acute respiratory distress and, in the worst case, death.

Nitrogen dioxide is an irritant to the eyes and respiratory tract and can cause inflammation of the airways, reduced lung function and a weakened immune system with increased susceptibility to respiratory infections (WHO, 2010). Even concentrations around half the exposure limit can be irritating to the airways and eyes. Nitrogen dioxide affects asthma and allergy sufferers by increasing sensitivity to allergens (substances that cause allergy or hypersensitivity). Tests have been carried out in which allergy sufferers reacted to a very low dose of an allergen that would not normally affect them after exposure to nitrogen dioxide.

OEL values and guidelines

In 2019, the Swedish Work Environment Authority's hygienic limit values for nitrogen dioxide were significantly reduced. The OEL time-weighted average over an 8-hour working day has been reduced from 4 mg/m³ to 0.96 mg/m³ and the short-term exposure limit during a reference period of 15 minutes from 10 mg/m³ to 1.9 mg/m³. The OEL value for nitrogen dioxide is intended to take into account the cumulative effect of the substances present in diesel exhaust.

The World Health Organisation's health-based guideline for indoor nitrogen dioxide is 10 μg/m³ as an annual average and 25 μg/m³ as a daily average (WHO, 2021). In previous Swedish studies, mean indoor concentrations and mean population exposure to nitrogen dioxide have usually been below 20 μg/m³.

Where does it occur on board?

Sulfur dioxide (SO₂) is formed during the combustion of sulfur-containing fuel and is not normally formed in indoor environments. The level of sulfur dioxide indoors is largely determined by the concentrations in the outdoor air and is brought in with the ventilation supply air.

Health effects

Sulfur dioxide is a colourless gas with a pungent odour. It causes coughing, irritates the eyes and can cause inflammation of the respiratory tract, impaired lung function and a weakened immune system. In high concentrations (above 400-500 ppm), sulfur dioxide can cause respiratory problems and death in sensitive individuals.

OEL values and guidelines

The OEL time-weighted average over an 8-hour working day for sulfur dioxide is 1.3 mg/m³ (1 300 µg/m³). However, it does not protect asthmatics. Studies have shown that levels below half the limit value are required to prevent asthmatics from reacting. The World Health Organisation's health-based guideline for indoor sulfur dioxide is 40 μg/m³ as a daily average (WHO, 2021).

Where does it occur on board?

Carbon monoxide (CO) is formed by incomplete combustion. Carbon monoxide is colourless and odourless, making it difficult to detect. Measurements taken on ships during these projects showed that there were rarely detectable levels, either in engine rooms or in other spaces. The levels of carbon monoxide measured outdoors likely originated from exhaust plumes from the ship itself and from other ships in the vicinity.

Health effects

Carbon monoxide blocks the ability of haemoglobin to bind oxygen. Exposure to high levels can cause symptoms of angina in people with heart problems or even death. A study of the health effects of long-term exposure to carbon monoxide in steel workers also showed cognitive impairment, including fatigue and short-term memory problems. Carbon monoxide is an ototoxic substance. This means that exposure, even at levels just below the exposure limit, increases the risk of hearing damage from noise.

OEL values and guidelines

The OEL values for carbon monoxide have been lowered in line with increased knowledge of its health risks. The time-weighted average over an 8-hour working day is 20 ppm (23 mg/m³) and the short-term exposure limit is 100 ppm (117 mg/m³).

The World Health Organisation considers that long-term exposure to carbon monoxide levels must be well below the hygienic limit to be considered safe. The World Health Organisation's guideline for indoor environments is 4 mg/m³ (3 ppm) for 8 hours exposure (WHO, 2021).

Where does it occur on board?

Carbon dioxide is produced by combustion of fuel oil and decomposition of organic matter. On ships, for example, it can be found in the holds of wood, pellets or grain, and in sewage systems. Humans also exhale carbon dioxide. Measurements of carbon dioxide concentrations can therefore be used for assessment of the function of ventilation. The lower the airflow, the higher the carbon dioxide content.

Health effects

Carbon dioxide is a colourless, odourless gas. It is not poisonous, but because it is heavier than air, it can cause asphyxiation. The highest levels of carbon dioxide are usually found at the lowest point in a room. Levels above 50,000 ppm of carbon dioxide in inhaled air can cause unconsciousness and higher levels can cause suffocation.

OEL values and guidelines

Carbon dioxide is used as an indicator of good indoor ventilation. If the carbon dioxide level in a room under normal use regularly exceeds 1,000 ppm, it is an indication that the ventilation is inadequate.

Where does it occur on board?

In outdoor air, ozone is formed by photochemical reactions initiated by sunlight. In indoor environments, ozone is introduced from outdoor air through ventilation, but there are also processes in the working environment that can produce ozone, such as welding. Because ozone is a highly reactive gas, it is used for decontamination, odour removal and duct cleaning in commercial kitchens and restaurants.

Health effects

Ozone irritates the eyes, nose and throat, causes respiratory problems and lung disease, impairs lung function and can trigger asthma. Very high exposures can cause acute damage that can lead to long-term respiratory diseases such as asthma or chronic bronchitis.

OEL values and guidelines

The time-weighted average over an 8-hour working day is 0.2 mg/m³ (0.1 ppm) and the short-term limit value is 0.6 mg/m³ (0.3 ppm). With regard to ozone generators in catering environments, the Swedish Work Environment Authority recommends that there should be sensors in the kitchen that warn of high ozone levels. The ozone generating unit should also be interlocked with the exhaust air fan and cleaning hatches. All units, ducts and cleaning hatches should be labelled if they may contain ozone.

Where does it occur on board?

The main sources of volatile organic compounds (VOCs) are indoors. They are emitted from building materials, furniture, office equipment and cleaning chemicals, but also during cooking. On a ship, the main sources of VOCs are the ship's fuel, both through evaporation and as a component of exhaust gases, as well as paints and solvents.

To give an indication of the concentration of VOCs in indoor air, it is often given as the sum of all VOCs analysed, known as Total Volatile Organic Compounds (TVOC).

Health effects

The ability of organic substances to cause health effects varies greatly depending on the substance, from those with no known health effects to those that are highly toxic. VOCs contribute to odours that impair the perceived quality of indoor air. This can lead to headaches, eye irritation, respiratory problems and reduced performance. As with other air pollutants, health effects depend on many factors. Acute symptoms include nose, throat and eye irritation, headaches, nausea, dizziness and allergic skin reactions. In addition, some VOC compounds can damage internal organs such as the liver and kidneys, leading to chronic health risks.

Benzene is a volatile organic compound commonly found in fuel oils. It is easily absorbed into the human body, either by inhalation or through skin contact. Benzene can irritate the mucous membranes of the eyes and respiratory tract, cause tiredness and, in the worst case, damage the heart. Benzene is classified as risk group 1, carcinogenic to humans and may cause leukaemia.

OEL values and guidelines

There are no limit values for TVOC because it includes a large number of different VOCs with different properties. However, there are recommended guideline values for indoor environments (UBA, 2018). TVOC levels in indoor environments should be kept to a maximum long-term average of 200-300 µg/m³ or lower if possible. According to the UBA, the limit range between 1,000 and 3,000 µg/m³ should not be exceeded in indoor environments where people are permanently present.

The OEL value for benzene is set on the basis of its carcinogenic effect. From April 2026, the Swedish Work Environment Authority will further reduce the limit value for benzene.

Where does it occur on board?

Polycyclic aromatic hydrocarbons (PAHs) are a group of several hundred substances found in many fuels. PAHs are also formed when organic materials are heated or incompletely burned in the absence of oxygen. PAHs can be present in exhaust gases drawn in by the ship's ventilation system. They can also evaporate from fuels and are present in tobacco smoke.

Health effects

The acute and long-term health effects of PAHs depend mainly on the route of exposure (inhalation, skin contact or ingestion), the duration and the level of exposure. It also depends on the toxicity of the PAH compound and the general health status of the exposed person. Short-term exposure to high levels of PAHs can cause skin and eye irritation, nausea and reduced lung function in asthmatics. Long-term exposure to low levels of some PAHs, including benzo(a)pyrene, can cause lung cancer. Like diesel exhaust and benzene, benzo(a)pyrene is classified as risk group 1. It is one of the oldest known carcinogens for occupational exposure. As early as the 18^th century, it was discovered that chimney sweeps suffered from a specific type of cancer linked to their exposure to soot, and in 1933 benzo(a)pyrene was identified as a carcinogenic substance in soot and exhaust gases.

Naphthalene, another PAH found on board, is classified as possibly carcinogenic to humans (IARC Group 2B). Naphthalene is mainly produced by combustion processes. Exposure to naphthalene is usually through inhalation of exhaust gases, solvents, lubricants, tobacco smoke or consumer products such as hairspray.

Exposure to naphthalene can be irritating to the eyes and mucous membranes and can cause haemolytic anaemia, a condition in which red blood cells break down.

OEL values and guidelines

Because PAHs are made up of many substances with different health effects, there is no guideline value for total PAHs. Since the 1970s, methods have been available to measure and analyse 16 priority PAHs identified by the United States Environmental Protection Agency (US EPA). This list has been a basic reference for determining the levels and health risks of PAH exposure for a long time. In recent years, more substances have been added to the list of PAHs that should be analysed to further improve knowledge of exposures and potential health effects. Which individual PAHs are analysed depends on the area under investigation. However, it is common to include naphthalene and benzo(a)pyrene in all methods, as they are major contributors to the health effects of PAHs. Indoor air quality guidelines can be found in the WHO Guidelines for Indoor Air Quality (WHO, 2010).

Where does it occur on board?

Formaldehyde originates mainly from various indoor sources, such as building materials and furnishings. Formaldehyde is a component of adhesives used in the manufacture of particleboard and other wood-based composites. Formaldehyde is a common product of the conversion of other hydrocarbons in the atmosphere or in combustion processes. For example, it can be produced by the combustion of methanol in methanol-fuelled engines.

Formaldehyde has not been shown to be a problem on ships because working and living environments are much better ventilated than, for example, homes and offices.

Health effects

Formaldehyde is an irritant and is classified as risk group 1, carcinogenic to humans.

OEL values and guidelines

The OEL time-weighted average over an 8-hour working day for formaldehyde is 370 µg/m³ and the 15-minute short-term limit value is 740 µg/m³. The WHO recommends a guideline value of 100 µg/m³, averaged over 30 minutes. Other sources indicate a health-based long-term average (1 year) of 10 µg/m³ (Public Health England, 2019).