THINK DETAILS FOR TREE 4

          Is there a threat to users of the sea?

As a general rule, access to the casualty area should be limited and activities in the area restricted.  Restrictions should be lifted once the area is declared safe. The size of the risk area can be estimated by monitoring the air quality and/or using dispersion models. There are certain limitations when using either dispersion models or calculations, since certain peculiar behaviour patterns may arise due to specific atmospheric conditions or terrain roughness which may produce results different to the expected output of a model or from calculations.  Thus, results have to be treated with caution.   

In the absence of using a model, the following table provides crude estimates on the extent of the health risk in the downwind direction for gases (vapour pressure > 100 kPa) which are commonly transported.  In the case where the amount spilled falls between the quantities shown in the table, a decision would need to be taken on which quantity best represents the quantity spilled.

                                   Table:  Estimated extent of the health risk in the downwind direction for different quantities of gases spilled

The spread of an evaporated vapour from a liquid spill (fast evaporator) can be estimated by multiplying the values in the right hand column of the table by the ratio of VP/100 where VP is the liquid’s vapour pressure in kPa at ambient temperature.

            e.g.  for a 10 tonne benzene spill at 15-20^oC ambient temperature

                    (VP of benzene = 8 kPa at 15^oC)

            Distance of downwind health risk:  8/100 x 5000 = 400 metres

If it is decided to monitor, then an appropriate gas detection system for toxic gases would be used.  Some marine pollution control vessels are equipped with sampling and gas detection systems with data logging which perform the task of providing information as to whether and how severely the air surrounding the ship or the filtered air inside has been polluted, and whether this presents a hazard to the personnel on board or to the vessel itself, making it necessary to abort the operation.

For outdoor use, on board a damaged ship, portable gas monitoring devices can be used.  In instances that a portable monitoring device consisting of detection tubes with bellows or pumps is used, it is important to note that each chemical has a specific type of detector tube.  It is important to ensure that the shelf-life of a tube has not expired and to acquaint oneself with the instruction cards that accompany the detector tubes.  Outdoors, the concentration will have large fluctuations due to changes of wind direction and/or velocity.  For this reason, one should target a time interval (e.g. one minute) which will be a time representation of the external concentration.

Diagram:  Hazardous zone of a chemical cloud originating from a spill.  The outer limit of the zone is established by means of measuring devices

There is no widely accepted method for the selection of an appropriate exposure limit to establish a safe distance for response personnel.  The problem is that a concentration of a chemical which is barely detectable may have potential to cause considerable irritation or other damage.  Some options in order of decreasing preference for selecting an appropriate exposure limit are as follows:

·         consult a toxicologist or similarly qualified individual for advice based on a formal review of the toxicity of the material concern;

·         use the highest value among the following:

-          IDLH value divided by 10 (with “10” being a safety factor)

-          TLV-STEL

-          TLV-TWA multiplied by 3 (if a TLV-STEL is not available or does not exist)

-          TLV-C