In May 2005, two cases were reported of fluid bromine being released in schools. Further cases of bromine being released in schools in the last few years have been brought to attention as well. In the case in May, a teacher lost control over a small amount of bromine – it had slipped out of his hands, leading to the evacuation of the school.
We would like to briefly describe these cases as detailed by the mass media once again in this article. Then we would like to estimate how much time is required until the vaporisation of a bromine spillage results in a dangerous concentration indoors.
Case in Hanover, Germany
On the 12th December 2003, 30 Children with respiratory difficulties were taken to hospital after an accident occurring during their Chemistry lesson at a school in Hannover. They had to remain under observation there for at least eight hours. The Chemistry teacher had to be taken to a clinic as well.
During this incident, approximately one eighth of a litre is said to have leaked out, said a police spokesperson. The youths aged between 15 and 17 and the teacher complained about difficulties in breathing, headaches and dizziness. The fire brigade, together with a large contingent of about 60 catastrophe defence personnel, evacuated the school. The bromine was collected using a chemical binder. The reason why the substance escaped in the first instance was unclear. ‘Although the pupils left the room immediately, they naturally had still breathed in some of the substance,’ the fire brigade spokesperson said. Since bromine is heavier than air, it must have also reached the Physics room under the Chemistry room via the window or the air-conditioning sytem. Some 8th class pupils who had been sitting there also breathed in the toxic substance. 'However nobody was seriously injured. The pupils and the teacher had to remain in the clinic for 8 hours for precautionary reasons' commented the spokesperson. (fatnews.de)
Case in Bürglen, Switzerland
This incident took place on the 20th May 2005 at a (sixth form) school called Bürglen in Switzerland during a Chemistery class. A teacher lost control of a glass ampoule containing bromine which broke on the floor, releasing fluid bromine. The pupils and the teacher left the school building immediately. After the building had been thoroughly aired, the pupils were able to return to the school building that afternoon. No persons had been injured (according to the police announcement on the 20. 5. 2005).
Case at the University of Wisconsin – Stevens Point , USA
On the 28th May 2005, a professor dropped a container with its 50ml of fluid bromine. The building was then evacuated as a result. The catastrophe personnel, attired in full body protective gear, aired the building room by room using a propeller (see the 7WSAW Newschannel video). No persons were injured.
Effect of Bromine on Persons
Bromine (from the Greek ‘bromos’ = stench) can already be perceived by its smell in tiny concentrations (average odour threshold 0.05 p.p.m.). Bromine causes painful, deep wounds to the skin. It has a caustic effect on the bronchial tubes and also acts as a sedative in compounds i.e. is tranquillising. Previously it was used successfully as a substance for treating epilepsy. Bromine has been classified as being more toxic than e.g. chlorine. The AEGL- values (Acute Exposure Guideline Levels) 1 / 2 / 3 are for 10 minutes’ exposure at 0.033 ppm / 0.55 ppm. / 19 ppm. (see Acute Exposure Guideline Levels - U.S. Environmental Protection Agency).
Estimating the Air Concentration
Bromine exhibits a boiling point of 59°C and is present as a liquid at room temperatures. When estimating the air concentration one therefore assumes vaporisation from a leak.
Assumptions:
50 ml of Bromine is emptied, a circular leak with a diameter of 25 cm is formed.
The room has the following dimenstions: 5 m x 5 m x 2.4 m = 60 m³.
The air exchange rate in the room is 1/h.
The room temperature is 25°C.
The description of the dispersion of the substance in closed rooms with natural or artificial ventilation is difficult due to the manifold influences. In practice in air-conditioning technology, a simplified model is therefore consulted, which assumes that the new supply of fresh air totally mixes with the air in the room.
The evaporation rate is constant.
The dimension of the spillage remains constant.
The evaporated bromine mixes with the air in the room completely. In practice, the concentration directly above the spillage is at the highest. Since bromine vapours are more than 5x heavier than air, the bromine concentration near the ground is higher than that close to the ceiling.
The air velocity in the room is 0.3 m/s or 1 km/h.
The diffusion coefficient is determined using an increment method (-> 0.033 m²/h)
To estimate the evaporation rate we utilise a model according to J. Gmehling [Gmehling, 1989]:
G
Vaporisation rate [g/s]
D
Diffusion coefficient of Bromine in air [m²/s]
v
Kinematic viscosity of air [m²/s]
V
Room air velocity Raumluftgeschwindigkeit [m/s]
L
Length of the evaporation surface in the flow direction [Meter]
P
Saturation pressure of bromine [Pa]
R
Ideal gas constant [Pa m³/mol K]
A
Paddle surface [m²]
T
Temperature [K]
The vaporisation rate is directly proportional to the vapour pressure of bromine, the temperature and the size of the spillage surface. As the temperature increases, the evaporation rate increases because the vapour pressure increases faster than the temperature T in Kelvin as a divisor. The dependency of the bromine vapour pressure on the temperature is made apparent in the following graph (click to enlarge the graph). The temperature gradient has also been entered for comparison.
Vertical dispersion
The following video in Flash 8 Format displays the vertical dispersion of bromine. A small amount of fluid bromine is released by allowing it to drip into a glass cylinder container. The bromine vaporises. The intensive blue colouring, which rises from the bottom to the top, renders the bromine concentration visible.
For starting the video left click on the grey area:
Hazard Assessment
The vaporisation rate with the above assumptions is 186 mg/s. The spillage evaporates in approx. 14 Minutes. In 10 seconds the average bromine room concentration rises to approx. 4.8 p.p.m, in 10 minutes to approx. 265 p.p.m.
When we compare the data with the average olfactory threshold of 0.05 p.p.m. it becomes clear that bromine is easily perceptible. The toxicity value AEGL-1 /2 / 3 values for 10 Minutes are 0.033 / 0.55 / and 19 p.p.m. respectively. To refresh one’s memory: the definition of the AEGL-3 value states that AEGL-3 is the airborne concentration (expressed as ppm or mg/m3) of a substance above which it is predicted that the general population, including susceptible individuals, could experience life-threatening health effects or death. Airborne substance concentrations below the AEGL-3 value but above the AEGL-2 is the airborne concentration (expressed as ppm or mg/m3) of a substance above which it is predicted that the general population, including susceptible individuals, could experience irreversible or other serious, long-lasting adverse health effects or an impaired ability to escape.
The increase in the average bromine concentration and the comparison to the toxicity values require immediate evacuation. The video shows that the mixing within a room/space is delayed due to the heavy gas behaviour of bromine. This form of filling up a room is a fundamental time determining factor and has a positive effect on one’s possibility of escape.
Chemical 'Neutralisation' of Bromine
Due to its chemical properties, bromine can be converted to non-volatile sodium bromide using a soda/sodium thiosulphate (Na2S2O3) solution.
This conversion is rendered visible in the following video (Flash 8 Format) due to the decolourisation of the brown bromine at the bottom as well as in the bromine vapour.
For starting the video left click on the grey area:
Conclusion
Bromine is a very critical substance, which requires that swift and correct measures be taken after it has been released.
Literature
Keil C.B, Mathematical Models for Estimating Occupational Exposure to Chemicals, AIHA, Fairfax 2000
Gmehling J., Weidlich U., Lehmann E., Fröhlich N., Verfahren zur Berechnung von Luftkonzentrationen bei Freisetzung von Stoffen aus flüssigen Produktgemischen, Staub - Reinhaltung der Luft 49 (1989) 227-230
Poling B.E., Prausnitz J.M., O'Connell J.P., The Properties of Gases and Liquids, McGraw-Hill, New York 2000
Accident report by NewChannel7 an Universität von Wisconsin - Stevens Point, USA see wsaw.com
Accident report by fatnews.de in Hannover see fatnews.de
