FAQs

Tritium (symbol “³H”) is an isotope of the element hydrogen (symbol “¹H”). Gaseous tritium is a colourless, odourless, radioactive gas with the same chemical properties as hydrogen. A nucleus of tritium transforms to a nucleus of helium with the emission of a beta radiation of maximum energy 18.6 keV. The activity of tritium decreases by undergoing the transformation by half in every 12.3 years. Tritium forms compounds in the same way as hydrogen. The tritiated water, which has the same chemical properties as water, can be formed as a compound of tritium and oxygen with tritium replacing hydrogen in water molecules (symbol “H₂O”) in two common forms separately denoted by the symbols “HTO” and “T₂O”.

The tritium exit sign is a self-luminous product illuminated by gaseous tritium light sources (GTLS) for the indication of the location of fire emergency exit. Each GTLS is a glass capsule filled with the radioactive tritium gas made to a form resembling fluorescent tubes. The inner surface of the glass tubes is coated with luminous phosphor. The beta radiation from the disintegration of tritium induces the emission of light from the phosphor when they hit the wall of the tubes. This phenomenon of induction of luminescence by radiation is known as “radioluminescence”. The intensity of the light diminishes as the tritium in the tube decays. If there has been no leakage of tritium, the useful life of a GTLS is typically 10 - 12 years.

The quantity of tritium contained in each tritium exit sign varies with the size of the sign. The tritium exit sign used in Hong Kong may contain tritium with total activity ranging from 0.3 to 0.8 TBq (300 to 800 billion Becquerel).

Tritium poses limited radiological hazards if outside the human body because the beta radiation released during radioactive transformation does not have enough energy to penetrate the outer layer of dead skin cells. However, there are significant internal radiation hazards when tritium is taken into the body through inhalation, skin absorption, ingestion and injection. Inhalation is primarily a health concern in close proximity to a point of release, or in a confined or poorly ventilated location.

Tritium in the form of tritiated water has near to 100% uptake through inhalation and skin absorption. It poses much greater health hazards than exposure to the gaseous elemental form of tritium. Ingestion of tritium may occur if an individual has handled a device that has external contamination and then eats, drinks, applies cosmetics or touches the mouth or lips before washing hands. Injection may occur if an individual handles a broken tritium device and, unfortunately, the component of the device penetrates in the skin and force the tritium entering into the body. Inside the human body, the tritiated water will be uniformly distributed and the beta radiation emitted from tritium will continuously irradiate the surrounding tissues until the tritium atoms decay or are removed from the body through biological excretion. The potential health hazards increases directly with the dose of intake.

The potential radiological risk of a tritium exit sign mainly arises from a damaged sign, which may lead to accidental uptake of the released radioactive tritium gas by the human body, through inhalation, skin absorption or wound. If a tritium exit sign is used in its normal condition, the radiological risk is low and special radiological protection measures is not necessary.

A common conclusion of the published assessments of reputable agencies and authorities is that the potential dose commitment in a tritium sign rupture event is significant from a public health policy perspective. It is accordingly reasonable and proper for a broad system of oversight and control to address the probability of occurrence of sign breakage events.

The control on the use of tritium signs varies between different countries. Some of the information is given as follows:

In Mainland China, tritium exit signs are primarily used in mines, tunnels, oil fields and refineries, and dangerous goods stores. All end-users of tritium exit signs are required to be covered by licence. In Taiwan, end-users of the gaseous tritium light devices (GTLD) are exempted from the requirement of licence only for devices that contain no more than 300 GBq tritium.

In the USA, end-users are covered by general licence. The transferor is required to notify the Nuclear Regulatory Commission (NRC) or State Authority of the Agreement State as applicable within 30 days of transfer. The NRC has been very concerned about the conduct and practices of distributors of tritium signs in promoting their products.

In Canada, end-users of GTLD are exempted from the requirement of licensing under specific design requirements of the tritium sign with compliance to the relevant standards.

In Singapore, end-users of GTLD are required to apply for licence and register the premises if the tritium activity in individual device exceeds 70 GBq.

In New Zealand, end-users of GTLD must register their premises with the authority if the tritium activity in individual device exceeds 100 GBq. Each case would then be assessed on an individual basis as required. Furthermore, the New Zealand Building Code provides that ‘self-luminous signs (i.e. tritium signs), although permitted, should only be used where it is impracticable to use anything else’.

In Ireland, end-users of GTLD with tritium activity below 1 GBq are exempted from the requirements of registration and licensing.

In Finland, the use of tritium signs is generally not allowed.

In Sweden, tritium signs are only approved for use in aeroplanes.

UK has a relatively complex system. As prescribed in the Radioactive Substances (GTLD) Exemption Order 1985, exemption from the requirement of registration in respect of keeping and use of GTLDs of up to 30 TBq is permissible under specific requirements of the signs and installation conditions. On the other hand, the report Transport of Consumer Goods containing Small Quantities of Radioactive Materials published jointly by the National Radiological Protection Board of UK and the Gesellschaft fuer Anlagen-und Reaktorsicherheit of Germany in May 2001 mentions that the use of GTLD as non-consumer products has been withdrawn in UK.

As a fire safety installation, tritium exit signs are installed and used in indoor public areas usually occupied by members of public. In most circumstances, their presence is most likely unknown to the occupants of the premises. Loss, damage and breakage of tritium signs may easily be overlooked for long periods of time. People at the scene of damage, breakage and vandalism are most probably unaware of the potential risks that they are exposed to. There are also concerns over the potential improper management and disposal of disused signs especially when management is changed or when the premises are to be renovated or demolished.

The Board’s policy regarding tritium signs in Hong Kong is informed and justified by reference to a wide range of considerations. The Board’s licensing and exemption policies on signs containing tritium are as follows:

Licensing Policy

1. The use of tritium signs is only justified by the potential benefit of saving of life and where the use of electrical power is not possible or feasible;
2. if the aggregate activity of tritium in signs that are securely fixed at specified locations inside any specified premises exceeds 1,000 GBq, the end-user / possessor of tritium signs shall be bound by licence so that the status of management and condition of the signs are reassessed annually; and
3. importers, manufacturers, producers, distributors and sellers of tritium signs should be subject to licence requirements so that the health and safety of workers and directly affected members of the public will be adequately protected and the transfer and disposal of tritium signs will be kept under proper management from “cradle to grave”, so as to minimise the potential exposure of indirectly affected members of the public to tritium.

Exemption Policy

Subject to (a) being fulfilled and if the aggregate activity of tritium in signs that are securely fixed at specified locations inside any specified premises is 1,000 GBq or less, user / possessor of tritium signs may be granted an exemption which is renewable triennially so that the administrative burdens to the Board can be reduced without unduly compromising the health and safety of the public.

No local disposal of tritium sign is permissible. All such signs are to be disposed of by sending them to the supplier or manufacturer outside Hong Kong in accordance with the relevant licence or exemption condition stipulated by the Board when they can no longer serve the dedicated purpose of usage.

Although the Board has the view that tritium exit signs should be and in principle designed to be intrinsically safe to workers / end-users and to withstand severe handling and environments and, during the normal course of usage, the external radiological hazards are small, the following potential radiological risks of the tritium signs remain :

1. the potential health risks arising from the leakage of tritium from tritium signs installed in the same premises during the normal course of usage; and
2. the probability of events that might result in significant potential radiation exposure to the public when the signs are inadvertently or intentionally broken or improperly disposed of.

In view of that, the Board has taken consideration of the following key issues for the tritium exit sign policy:

1. The Principle of Justification for the introduction of a radiation exposure situation recommended by the International Commission on Radiological Protection (ICRP), which requires that ‘any decision that alters the radiation exposure situation should do more good than harm’. The potential benefit of tritium signs is the possibility of saving of life by providing illuminated indication of emergency egress routes in indoor premises where the use of other types of luminaries is judged to be impracticable. The potential harm of tritium signs is the possibility of increasing the internal radiation exposure of the uninformed public by (i) the leakage of tritium during the normal use of tritium signs and (ii) potential internal radiation exposure of the public on breakage of signs in the course of accidents, acts of vandalism and inadequate management leading to loss and improper disposal.
2. The shared view of reputable studies that the use of tritium devices should be limited to places where it is impracticable to use anything else.
3. The regimes of control of tritium devices in different countries and places. The approach adopted by many regulatory authorities is that the use should be limited to places where it is impracticable to use anything else.
4. The assessment of potential dose commitment to the exposed members of public in a sign breakage event, as reported by various independent bodies. The assessments show that the potential dose commitments are of considerable significance from a public health perspective.
5. The availability of other types of luminaries that do not contain radioactive substances such as LEDs and photoluminescence devices. As the licensing authority with the duty to protect public health from the potential deleterious effects of ionising radiations, the Board has properly considered the availability of these alternatives in the overall balance between the risks and benefits of using tritium luminaries.
6. Whether the use of radioactivity for the illumination of emergency egress is indispensable.

The licence / exemption applicant should provide reasons and necessary information, with supporting documentation (e.g. floor plan, photos, electrical supply circuit diagram, etc. of the proposed sign locations), of choosing tritium sign against other types of exit sign for the proposed location of installation. The Radiation Board will carefully consider all factors based on the Justification Principle, among which the possibility or feasibility of electrical power supply is one of the considerations. Circumstantial factors like building structural constraint on installing electrical wiring around the emergency exit, difficulty in using electricity in extreme places like tunnels, mines and quarries or locations where inflammable or explosive gas is present are also factors that Radiation Board will take into consideration.

1. Licences may be annually renewed up to the end of the working life of existing tritium signs (as specified by the manufacturer and marked on each tritium sign).
2. In case a tritium sign is lost or damaged before the end of its working life, it may be replaced by a new tritium sign if immediate replacement with non-radioactive alternatives is not possible. However, the use of such newly installed replacement tritium sign shall only be permitted for a period covering the remaining working life of the lost/damaged sign. Justification is required for its use beyond such period.
3. Justification of use is required for installing new tritium signs as replacement of the expired signs. Failing to justify the use, the expired tritium signs shall be replaced by non-radioactive alternatives.
4. Justification of use is required for installing any additional tritium sign.

The renewing of licence is conditioned on full compliance of the Radiation Ordinance and the conditions prescribed in the licence.

The licence conditions for possession and use of tritium exit signs require that each tritium exit sign shall bear a clear notice both in English and Chinese indicating the presence of the radioactive substance. The form of the notice endorsed by the Radiation Board is given below:

The notice, of dimension no less than 1.5 cm in height, is to be displayed at the lower edge of the tritium sign housing with the following content in both English and Chinese:

Caution - Radioactive Substance. Do not dispose of as normal trash.

注意 - 放射性物質。不可作普通廢物處置。