1-Bromopropane

Something you don't understand? Look it up on the Environment and Energy Wiki Encyclopaedia (beta version)

The Environment and Energy Portal is now operational

This substance, also commonly known as n-propyl bromide and usually abbreviated to nPB, has been one causing much controversy for 10 years. As a solvent, it is good, with characteristics similar to TCA, TCE and close to CTC. However, it has a property that sets it apart from any other OD solvent, a short Atmospheric Residency Time. This has created a situation that may be described as chaotic.

ODP of nPB

All the other OD solvents have Atmospheric Residency Times (lifetime) counting in years. This means that they become homogenised throughout the atmosphere. nPB has a lifetime counting in days, variously quoted at 12 to 25 days. This makes it unlikely for much of it to reach the ozone layer before it is destroyed. Unfortunately, what little there is that does reach it is very effective at causing depletion. Because of these two facts, the usual models for calculating the ODP are not valid.

To make things worse, the main cause of destruction of the nPB molecule is reaction with free hydroxyl radicals in the atmosphere, and the concentration of these is variable according to place and weather, so it is difficult to forecast the lifetime for any emissions at a given place and time. Also, the various chemical compounds formed during this hydrolysis may also be somewhat ozone-depleting and some may have longer lifetimes than nPB itself.

Furthermore, the transport from the place of emission to the ozone layer is not constant. In each of the earth's hemispheres, there are three convection mechanisms, called cells. The most violent one is called the Hadley Cell and this flows from latitudes of about 20 - 25° N (or S) of the current zenith latitude of the sun towards the zenith at ground level, causing the Trade Winds. When it reaches the Intertropical Convergence Zone (ITCZ), between about ±5° from the zenith latitude, the convection currents are directed upwards to the tropopause, typically at 15 km altitude in the ITCZ and already at the bottom of the ozone layer. From there, the current flows away from the ITCZ again and, eventually, downwards, to close the circuit. To add a little spice to this, the ITCZ is notorious for tropical thunderstorms, with violent updraughts that can push the tropopause upwards to altitudes of 18 or even 20 km.

Any nPB vapour or equally ozone-depleting decomposition substances caught in the Hadley Cell movement will be transported to the tropopause before its natural decomposition is complete. There are also mechanisms which can interchange tropospheric and stratospheric air across the tropopause, so a small percentage of the released nPB molecules may even reach the upper ozone layer, where they can cause the most harm.

Similarly, there can be transport to the tropopause within the other two cells, the Ferrel Cell and the Polar Cell. The Ferrel or mid-latitude Cell is somewhat less important as being less violent and its top, at about 12 km, is lower than the bottom of the ozone layer. As the winds are essentially more westerly, the time of circulation is much longer. Less emitted nPB vapour in the mid-latitudes is thus less likely to reach the ozone layer. The Polar Cell is even less likely to cause nPB emissions to reach the ozone layer, because the tropopause ceiling is typically only 8 - 10 km.

From this, it can be seen that the effect of nPB emissions on the ozone layer can vary enormously from place to place, according to the season and even the weather. It is therefore impossible to give it an ODP figure, necessary before it may be deemed a controlled substance under the Montreal Protocol. To make things worse, some places with expected large emissions, such as S. Florida and Gangzhou Province, are expected to have enormous seasonal variations of effective ODP, as they lie within the ITCZ in summer, but not in winter.

Various persons and bodies have calculated the probable average spot ODP from emissions in a few places. Broadly, these have mostly been in the range of 0.005 to 0.125, the latter from tropical countries, with about 0.025 in mid-latitude countries. These values all lie within the range of ODPs of substances that are already controlled (some controlled HCFCs have ODP ranges starting at 0.001, up to halon-1301 with an ODP of 10). It would therefore seem logical that nPB should be a controlled substance.

The Parties to the Protocol charged the Scientific Assessment Panel with calculating the overall effect on the ozone layer over 50 years, based on probable emissions from various countries. This is a costly and long task as new models have to be calculated involving many variables that have never before been considered. An ad hoc task force (which I had the honour of co-chairing) provided these experts with probable and maximum emissions of nPB by country and state under a number of scenarios, along with many other data relative to the substance. This work has yet to be completed.

Health and Safety Issues with nPB

nPB is a toxic solvent. Its acute toxicity is low, but its sub-chronic and chronic toxicity have not yet been fully established.

Laboratory experiments on animals have demonstrated both reproductive and neuropathic effects at medium exposure levels. It must be realised that these test results cannot be extrapolated to humans who have a different metabolism to halocarbons, but they can provide an indication of areas where to look for problems.

In many countries, a toxic chemical can be freely sold and used until it can be proved to be harmful. The manufacturers may recommend an Operator Exposure Limit (OEL) without this being legally binding. To this day, even 10 years since it was proposed on the market as an industrial solvent, no country has proposed a mandatory OEL. Like for calculating the ODP, this has proven to be controversial. Even today, the recommended OEL from different manufacturers vary from 5 ppm to 100 ppm. This is an enormous range and is a result of ignorance and the risks that manufacturers may wish to take to boost their commercial sales. The problem is that no one really knows what is a safe OEL.

Up to a year or two ago, more attention was paid to the eventual reproductive toxicity effects. Most recommended levels were based on this, the neuropathic effects being considered less important. However, various anecdotal reports have come in over the past few years of people suffering neuropathic effects on the Central and Peripheral Nervous Systems, some of them possibly permanent. These have been insufficient in number to make a serious epidemiological study and some cases have certainly involved high exposure levels. Notwithstanding, there are sufficient data to consider that neuropathy as a consequence to exposure to nPB should not be taken lightly. In my own experience, the operators in one factory I visited complained of headaches at the end of a working day, with nPB exposure levels of 3 - 5 ppm, whereas they previously had no problems with CFC-113 at much higher levels.

The ACGIH, a respected US NGO, recommends an OEL of 10 ppm. The State of California recommends and proposes legislation for an OEL of 1 ppm, but this is almost ridiculous.

The SNAP saga

In the USA, the Environmental Protection Agency runs a programme to recommend suitable substitutes for ozone-depleting substances, called SNAP. Normally, a substance should be proposed for approval for use, or otherwise, within 6 months of application. Several companies applied for SNAP approval for nPB in 1996. Every time that approval seemed imminent, some obstacle appeared and, each year, the US EPA has stated that it is coming. In 2003, a proposed rulemaking was published in the US Federal Register (with a proposed 25 ppm recommended OEL, with some other restrictions). It was stated that this would become definitive in February 2004, then August 2004. We are still waiting. In the meanwhile, nPB can be used without restrictions in the USA. Will this ever end?

My recommendations

nPB is ozone depleting. nPB is toxic. We don't know enough about it in either respect. I would therefore recommend that nPB be not used if there is another solvent or process which would do the same job (and there nearly always is an alternative). If it is used as being essential, then emissions should be minimised, so that little is allowed to escape to the atmosphere or the workplace. I recommend an OEL of 10 ppm, which is feasible for most applications but may require expensive equipment, 25 ppm at the outside.

In particular, I strongly recommend that nPB never be used as a substitute for the non-ozone-depleting chlorinated solvents, such as methylene chloride, trichloroethylene and perchloroethylene, which have a century of safe use within industry, with adequate precautions.

In other words, I recommend the Precautionary Principle. Play safe to protect the environment and those who have to come into contact with the solvent.

Discuss anything related to this page at this forum.

The opinions on this site are personal and do not necessarily reflect those of any third party. All information is given in good faith but no responsibility is taken for such information; any person or organisation using such information should ascertain that it is suitable for his/her/their conditions of use.

No reproduction of the contents of part or the whole of this site may be made in any form without the written permission of the owner. An exception is made that a print-out may be made for one individual's private use without seeking permission; it is forbidden to make multiple copies or to photocopy a print-out. Links may be made to this site.