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Initial isolation and protective action distances


THE TABLE ON SUGGESTED DISTANCES FOR ISOLATING UNPROTECTED PEOPLE FROM SPILL AREAS INVOLVING HAZARDOUS CHEMICALS SHOWN ARE FOR LISTED CHEMICALS WHEN THEY ARE NOT ON FIRE.

Only a few chemicals of the Motor vehicle Rules, 1989 and whose vapours have the potential to produce toxic effects have been selected. The table 6 is useful for no more than 80 minutes of an incident involving these hazardous chemicals. It is suggested that the use of this table be limited specifically to the initial phase of a NO FIRE spill incident during transport.

The smallest ISOLATION DISTANCE indicated in the table is 45.7 meters and the smallest PROTECTIVE ACTION DISTANCE is 320 meters. This means that 45.7 meters is the minimum distance for isolating the unprotected public for any hazardous chemical spill that is listed in the table. Distances for both small spills and large spills are shown in the table and the largest distance for protective action is 8 km. The approximate distance and air borne release would travel in 30 minutes with a wind speed of 2.68 m/s. Some releases could require protective actions beyond 8 km after the first 30 minutes following the initial release.

This module assists the first responders in identifying hazardous chemicals and provides advice for initial emergency action. It is not an excuse for a responder to try to control an accident alone. Instead, the first responder will take action to and move an unprotected person out of the danger area.

If a hazardous chemical cloud goes between several multi story building or a down a valley the cloud may affect people much further away from the distances specified and protective action distances should be increased accordingly. Fire involvement in an additional leaking tank also indicates the need to increase the isolation and protective action distances.

For hazardous chemicals listed in the table, if a fire begins and burns the spill chemical, the toxic effects may become less important compared to the fire, explosion or BLEVE hazards. For flammable chemicals the potential fragmentation hazards i.e. formation of thermal degradation product usually requires a greater isolation area in all directions despite any shorter distances suggested in the table.

Regardless of the number of meters that have been indicated if unprotected people are being affected the isolation and protective action distances should be increased.

Every hazardous chemical incident is different. Each can have special problems and concerns. Actions to protect public must be selected carefully. This module can help with a initial decision on how to protect the threatened public. Officials must continue to gather information and monitor the situation.

Evacuation is by itself, a process with significant risk for the persons being evacuated. Therefore, protecting in place should always be a first consideration If the buildings can be sealed tightly from any fresh air entering. In the case of short term spills and cold toxic vapour clouds the hazardous chemicals may be deflected or reflected by the multi-story buildings and pass by without affecting the occupants of the buildings. People in the upper floors of a tall rise building within the downwind protective action distance may often be safer by remaining where they are. Air handling equipments if any, like air conditioners, etc. should be shut down to keep out hazardous chemical vapours.

It is vital that communication is constantly maintained with competent persons inside such buildings so that they could be advised of the changing conditions. Those persons protected in place should be warned to stay far from a window in direct line of the scene of the incident.

BACKGROUND INFORMATION

Based on a Gaussian dispersion model, isolation and protective distances have been predicted. A small spill refers to an accident with a container size equal to or smaller than a 248 kg drum (55 gallons). Large spill refers to a one ton cylinder a tank truck or rail car. Predictions are for neutral weather stability (class D), overcast night with wind speed of 2.68 rn/s and a temperature of 35°C. LC 50 values were used to estimate short term exposure limits wherever TLVs STELs or TLV-TWAs were not available.

HOW TO USE THE TABLE

  1. Responders should have:
  • Identified the material by its ID Number and Name;
  • Noted the wind direction.
  1. Now responders should look in this table for the ID Number and Name of the chemical in this incident. Some ID numbers have more than one shipping name listed. Look for the specific name of the chemical.
  2. If respond find a matching entry in the table, use the following steps (3 to 6) to determine the area requiring immediate protective actions.
  3. Determine if the incident involves a SMALL or LARGE spill. Generally a SMALL SPILL is one which involves a single, small package (i.e. up to a 200¬248 kg or one drum), small cylinder, or a small leak from a large package. A LARGE SPILL is one which involves a big spill from an opening in a large package or multiple spills from many small packages.
  4. Look up the initial ISOLATION distance. Direct all persons to move, in a crosswind direction, away from the spill to the distance specified.

Next, look up the initial PROTECTIVE ACTION DISTANCE shown in the table. For a given hazardous chemical and spill size, the table gives the distance in meters downwind in which protective actions should be considered. For practical purposes, the Protective Action Zone (i.e. the area in which people are at risk of harmful exposure) is a square, whose length and width are the same as the downwind distance shown in the table.

 

Initiate Protective Actions to the extent possible, beginning with those closest to the spill site and working away from the site in the down wind direction.

The shape of the area in which protective actions should be taken (the Protective Action Zone) is shown in this figure. The spill is located at the center of the small circle. The circle represents the ISOLATION zone around the spill.

If the material is on fire or explosion, there may be toxic combustion products.

Table
Initial Isolation and Protective Action Distances
(in metres)

 

 
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