Risk calculation and fire safety.  How do we evaluate the (fire) risks that threaten us?   A risk can be defined as the unwanted occurrence with damaging results. It can be characterised by the probability of occurrence and the severity of the consequences.   The probability can also be subdivided into two factors:  The frequency of occurrence of the undesirable fact, such as the start of a fire in a building  The duration of exposure to the effects of this fact (how long do we stay in the building?) The severity can also be divided into two factors:  the seriousness of the consequences ( nuisances, small injuries, major damage, death)  the number of possible victims ( one person, a group, everybody) The mechanism of risk acceptance. People accept risks even with deadly consequences if the combination of probability and severity is low enough. For this reason, risk calculation expresses often this acceptable risk level as a number that combines a frequency rate with a severity degree. One can also visualise risk in a profile with two axes: one to measure the probability or frequency, the other to indicate the level of the consequences. However, there is no fixed boundary between acceptable and unacceptable risks. Sometimes people are obliged to accept a risk because they do not have the means to protect themselves or because they consider these risks as inherent to life. In this way, people accept in some countries floods or earthquakes as part of their living conditions.   One tends to reduce the probability of occurrence when the consequences are more severe, when more people are exposed to the risk at the same time, and when the duration of exposure is lengthy.  A risk is considered less acceptable when the consequences are readily visible.   A risk is more easily accepted when the consequences are reversible, of short duration or repairable, when there is a visible benefit in taking the risk, and when one thinks to have control of the causes of the undesirable event.   When all these elements are taken into account one can perfectly explain why a lot of people prefer travelling in their own car above flying, even when the air travel is cheaper: A serious car accident will eventually give 5 deaths, a aeroplane crash will cause a hundred of victims; during a car trip, the danger is only perceived for short moments, but one will fear for the crash during the whole flight. In our car, we think that we have control of the situation as we choose the course of our trip. In such a situation, an air voyage must be much safer than a car trip to give us the same perception of safety.   An unknown or hidden risk will be less acceptable. In this way, we have more fear of a fire during the night than during daytime, although the probability is much higher when we do all kind of things, then when we are at rest.   There are plenty of statistics available to indicate us what level of frequency and severity of accidents are existing and what are the socially accepted levels of safety. The probabilistic approach of risk is widely used in the chemical and nuclear industry and for work accidents, but is almost unknown in approach of safety of buildings.   The existing safety or acceptable risk level.   As there are very few people that are worried about the existing fire risk in our society and as there is no public discussion on unacceptable situations in this field, one can suppose that the present level of fire safety complies with the expectations of our citizens: We consider that the available level of fire safety in our (West- European) society is acceptable.   So, what level of safety do we have reached?   Usually the basic acceptable level of risk is defined in a situation of permanent exposure, i.e. that the danger and the risk are always together. Such a risk will be accepted when the probability of an accident with one death is less than 1 per million of persons per year, or 1.10-6 / person*year. Some studies and evaluation methods indicate that the acceptance of risk is reduced by the square of the number of possible victims: for 3 deaths, the acceptability is 10 x less, for 10 victims, it is 100 x less, and for a 100 deaths, it is 10.000 x less. This explains easily the much higher safety requirements imposed on nuclear industry and aviation, and makes it understandable why our safety rules are less stringent for low buildings compared to high rise buildings, that are less easily evacuated. For an accident with serious but reversible consequences (people injured), the acceptance level is about 1 per 10.000 persons per year, or 100.10-6/ person* year. If there is a direct benefit in taking the risk, the acceptance is at least 10 x higher. This explains e.g. the existing tolerance for road accidents, for Belgium almost 1000 deaths per year in 2002-2008 (= 100.10-6 /person*year). If the exposure is not permanent, the risk will be more easily accepted. Such is the case for the fire risk: In most European countries, the number of deaths by fire is per million habitants per year. This level of 5 * 10-6 deaths per person* year is 5 times higher than the basic level of permanent exposure but 40x lower than the road traffic risk. The acceptable level of fire risks.   For the risk of fire damage to our homes, we can make the following calculation: Of the average of 13.000 fires per year in Belgium, about 10.000 are residential fires. With an average surface area of homes is 160 m², and 400 million m² total habitable area of Belgian homes, the probability of a residential fire in Belgium is 25. 10-6 / year.m². This numbers is also valid in various other European countries. With an average of 4 persons per house, the probability of a person threatened by a home fire is = 1 /1000 per person per year. With 5 victims per million inhabitants, this means that the probability that one cannot escape from a residential fire is about 0.5 % of the incidents.   In Bern, Switzerland, the property insurance is mandatory with the Canton insurance company, which collaborates tightly with the fire brigades. Because of this particular situation, detailed statistics are available. These indicate that in 40 % of the cases the fire is already extinguished before the arrival of the fire brigade, for 90 % the fire is limited to the room of origin and only in 8 % there is a developed fire (flash-over), which is still controlled by the fire brigade in 2/3 of the cases. We can estimates that the situation is not significantly different In Belgium, and therefore conclude that the accepted level of property damage (total destruction of a house of 160 m²) is about 40 10-6 /year. This level is equivalent to 40 % of what is usually accepted for a risk with heavy but repairable consequences (= 100. 10- 6). This higher level of requirements can be explained by the direct nuisance caused by a residential fire.   In summary, we can conclude that we are using the following levels of safety for our houses:   the probability of death shall be less than 5.10-6/person.year the probability of total destruction shall be less than 40. 10-6 /year The levels of acceptable risk are not identical for the people and for the property, which is easily explained as the impact is different. More usual safety levels. As indicated in the previous observations it is possible to deduct additional safety levels for other fire scenarios, using a distinct reasoning for the safety of property and people. One can imagine that the required level of safety is higher for an apartment building due to the presence of more people. In the same manner, the reduced mobility and awareness of people in a school or a hospital will increase the required safety level, as a fire could result in more victims. A scenario with 10 victims is perfectly imaginable; reducing the acceptable risk by a factor is 100.   The risk level must also be much lower when the users of the building have no control on the danger such as in buildings accessible to the general public, but, on the contrary, it can be higher if the fire risk is largely linked to the presence of the users, such as in offices and factories. The risk of fire in buildings without people in it is directly linked to the importance it has for its user and to the exposure to adjacent buildings. The authorities shall take care of the interest of the neighbourhood: No one should suffer damage from a fire on other premises. They shall formulate their requirements in such a way that propagation of fire is prevented, that there are no victims to be expected with the users or the fire services, and that no irreversible damage shall be done to the environment.   The authorities will in the first place try to guarantee safe exit of the fire zone to all users. When there is no threat to the neighbours, or when is also no compelling need for the fire brigade to extinguish a fire, the authorities can in practice accept that occasionally a building is completely destroyed by fire.   If there is no direct threat for the neighbourhood, because of safety distances and fire walls, but there is a necessity for the fire brigade to stay for a short time inside the building, e.g. for checking an evacuation, the mandatory requirements will have to guarantee the safety of the firemen during that operation.   But when there is a direct threat for the neighbours, such as in an urban zone, or when the fire brigade has to operate for a longer time inside a building, to organise rescue operations or to control an environmentally dangerous fire, the level of safety required will be very close to what is expected for single family homes.   The owner or user of a building can agree with the safety requirements of the authorities or choose a higher safety level to cover better his own wishes to maintain and protect his property and/or (business) activities.   One can imagine that the expression of the required safety levels, based on the probability of occurrence, the degree of exposure to the risk and the magnitude of the potential consequences, will be a significant step forward on the way to performance based requirements. There is however, a major problem with statistics, which are in some way incomplete, and prone to different interpretations. Some countries will group residential buildings and offices in one category, other will have them separated. Some consider storage and manufacturing as a single activity, while others make the distinction. The source of information will colour the data: Fire brigades count the number of calls and victims, insurance the number of cases they will have to pay for, excluding the minor incidents. It is a logical to guess that the probability of a fire will be higher in industry than in residential buildings, because of the higher fire loads and the higher number of ignition sources available in industry. Yet, the number of calls to the fire brigade per m² de area is lower in industry, and on the other hand there are as much calls for storage as for homes, even if the number of ignition sources in storage is very low. One possible explanation is that in industry a large number of small fires is controlled by own means, without calling the fire brigade, while in storage the threat of a large fire is such that nobody hesitates to call in the public service.   It is therefore not easy to define an absolute and reliable risk level based on statistics alone, but is it is much easier to define a relative safety level, comparing any given situation with the residential risk which is well known and accepted. The acceptable risk level for a residential fire is based on a fire scenario that happens in a house of incombustible construction in an urban zone: The probability of a fire is low. At the start, the fire will develop slowly and can be rapidly be detected and signalled to the fire brigade. The public service can attack the fire before flashover has occurred and they can limit the damage to the room of origin in 90 % of the fires. The occupants have a 99.5 % chance to escape from the fire by their own means or to be rescued by the fire brigade.   This scenario does not consider any provision for preventive or protective measures, but does not consider either aggravating circumstances, such as access difficulties, etc.   The required safety level in other conditions can then be defined by applying correction factors that take into account:  the relation between frequency and severity the subjective perception of risk the available degree of protection PRINT  THIS SECTION  (pdf)