Calculating all of this information by hand at the time of a chemical incident would be too time-consuming to help a response, even if the firefighter or emergency responder had the training and education to make all of these calculations. Fortunately, the work and research has been already done for them.
The Environmental Protection Agency, the National Oceanic and Atmospheric Administration (NOAA) and the National Safety Council have developed software that aids in modeling chemical spills. CAMEO (Computer Aided Management of Emergency Operations) is the name for the comprehensive computer software program developed by the US Government and others. ALOHA is a component of this. (For more about CAMEO, and about how to acquire the software, including ALOHA, go to http://response.restoration.noaa.gov/chemaids.html, or contact your SERC or LEPC.)
ALOHA, Areal Location of Hazardous Atmospheres, is an air-dispersion model used to evaluate hazardous chemical scenarios and determine the likely "footprint" of such spills. ALOHA helps planners make comparisons, develop optional spill scenarios, and help them visualize what might happen. Many clouds of chemical vapor are colorless, and ALOHA is especially helpful in scenarios involving these chemicals.
ALOHA is not used with spills that will cause a cloud or plume that will be longer than six miles or ten kilometers. Another modeling software used for the Clean Air Act 112r Risk Management Program is available for downloading off the Internet at http://www.epa.gov/ceppo/tools/rmp-comp/rmp-comp.html. This software may be helpful to model a spill that will travel more than six miles.
Responders can use ALOHA as a response tool that can help them quantify what chemical dangers could be present. It can help responders with making estimations about how far a cloud of chemical vapor would likely go, and where. ALOHA links chemistry, toxicology, and meteorological data. ALOHA can help firefighters make an educated guess about what levels of the spilled chemical would likely present a fire hazard, or a health hazard, for example. The ALOHA program also can provide information about the levels of spilled chemicals that are likely to seep into buildings near the source of the chemical spill, and the program also has variables that allow firefighters to select the type of structures near the spill and the rate of air exchanges inside the buildings. (Air exchanges is the term for how many times in an hour the air inside a building is replaced by outside air. The key concept in shelter-in-place is to take measures to slow down air exchanges or to keep the chemical vapors from seeping into nearby buildings that are downwind from the spill.)
There are different rates of air exchanges in different types of buildings. ALOHA allows the user to enter in different air exchange rate data to make calculations, but it has settings already built into the program. Enclosed office buildings, sheltered or unsheltered, have an air exchange rate of 0.50 times per hour. This means that the air inside the enclosed office building is expected to change out twice per hour. For a single storied building sheltered by trees or shrubs, the rate of air exchange is 0.52 times per hour. For an unsheltered single story building, the rate is 0.62 times per hour. A double storied sheltered building has 0.37 air exchanges per hour, and 0.44 air exchanges per hour for an unsheltered double storied building.
ALOHA contains a site-specific database that incorporates the site's specific factors such as altitude, rates of incoming solar radiation, as determined by latitude and longitude, and stability class. Stability class refers to the amount of mixing of the air between the upper and lower atmosphere, and whether there is an air inversion. Air inversions tend to keep chemical gases from traveling and dissipating upwards, which generally causes the plume to travel a longer distance.
Unless the incident being modeled is occurring at a location listed in the ALOHA database, the latitude and longitude of the site must be entered. Firefighters and planners should be entering this data into the database of their computer system long before there is an incident. These are factors that relate to the place that a spill has occurred. If there is no breeze and the atmosphere is stable, the spill may form a circle around the site where the spill occurred. Low wind speeds or very stable conditions can cause patchiness and high concentrations of the spilled chemical near the spill site. A very stable atmospheric condition can lead to long thin footprints with high concentrations of the chemical far downwind. This is what happened in Bhopal, India. On the other hand, very breezy conditions can dissipate the chemical cloud quickly and minimize the hazards at a distance from the spill. ALOHA bases its modeling on spills that last less than one hour. Most spills are contained within that timeframe, but not all of them are.
ALOHA also contains the chemical data of over 1,000 pure chemicals. It even allows for a new chemical with its data and properties to be entered into the database. The rate that a chemical will evaporate is its vapor pressure. The higher the vapor pressure, the faster the evaporation. A liquid that will evaporate will turn into a far larger amount (cubic feet) of vapor than that of the original liquid. If a chemical being modeled is reactive, ALOHA will communicate this as a warning to the user. Some chemicals will have a second phase reaction that will cause the plume to behave in unpredictable ways. Chemicals under pressure behave differently, too. When there is a small hole in a pressurized tank of chemicals, there will be cooling of the tank, which may hinder the escape and dispersion of the chemicals for a period of time.
Other variable scenarios involve cooled liquids, which can boil once exposed to the ambient air, causing rapid and disastrous expansion.
Other site-specific data must be entered, and there is considerable variability allowed.
The ALOHA user must enter into the computer program the type of buildings in the affected area, which will be helpful later in determining the time it might take for a chemical to seep into the building and rise to unhealthful levels, as in the case of shelter-in-place. Another example would be if a flammable gas were released near buildings. ALOHA could help responders estimate if the level of the flammable gas in the building could rise to the point of causing a fire or explosion.
Another factor to be entered into ALOHA for it to make its calculations is the type of building surroundings that are at the site. Open terrain, usually in a rural setting, allows a plume of chemicals to travel further in every direction; forest or buildings may cause the plume to stand still or move slowly along a thin route, and keep chemical concentrations high. ALOHA defaults to the IDLH (Immediate Danger to Life and Health) for determining its "footprint" predictions, but sometimes there may be disagreement whether this is the correct standard. ALOHA allows for other values to be put into the modeling database.
ALOHA will ask how meteorological measurements were taken. Getting the weather report from the local airport is not entirely specific to the site where the spill has occurred, and airports take their weather measurements at a height relatively far above the ground, ten meters, where conditions may be quite different than ground level. Some firefighters have their own equipment to measure wind speed and direction, humidity, temperature, etc. which is usually taken at a lower elevation that airport data. At any rate, ALOHA requires for the data to be entered, including the source of the meteorological data.
ALOHA will ask for the source of the spill, and allows for four categories of source: Direct, Puddle, Tank, or Pipe. ALOHA will ask for specific information like the dimensions or area of tanks, puddles, diked areas, the size and shape of a hole, and temperature (if applicable). A spill through a pipe is the most restrictive source. Puddles are the least restrictive source, which explains why dikes are built around tanks of chemicals to control the size of the puddle that would form in the event of a spill. Firefighters sometimes have to construct dikes during a large spill for the same reasons.
ALOHA'S CALCULATION--THE "FOOTPRINT"
After the requisite data has been entered into ALOHA, then ALOHA will give the likely "footprint" of a spill.
It will show the distance of the plume, and a shaded area where it predicts the chemical will be at levels above the IDLH, or other levels entered into the computer. ALOHA's footprint will also show an area on either side of the plume that represents other areas that the plume could travel to. The center, shaded area is what ALOHA statistically predicts 19 out of 20 times the plume will be. The other areas on either side of the plume represent a 1 out of 20 chance of the plume going there. Heavy gases have wider areas of uncertainty than lighter gases.
The person using ALOHA can click on any point on the footprint map and get what ALOHA predicts the level of spilled chemical will be. The central plume has varying levels of predicted chemicals, with the highest concentration occurring closest to the source of the chemical vapor cloud.
Clicking on the lines of the footprint map will show other information. The dash line shows projected indoor concentrations of the chemical. This allows firefighters to better "guess" what levels of the chemicals people sheltered in place might be exposed to. Also, the firefighters may determine if gases seeping into buildings near the spill of a flammable chemical may have neared or reached the level that a fire or explosion is imminent or likely.
OTHER ALOHA INFORMATION
ALOHA maintains a text summary for any query throughout any modeling. This text summary can be printed out, and it includes other data, such as the molecular weight of the chemical, its TLV-TWA, IDLH, boiling point, vapor pressure, and ambient saturation concentration (the highest level the spill chemical will reach in the ambient air). The text summary also allows the user to see all of the data entered into ALOHA on one page to allow the user to make sure all the variables that have been entered are correct.
ALOHA has a Source Strength or Release Rate on its display setting where Footprints may be selected. The Source Strength setting will show what amount of the chemical will be released during a period of time. For a spill from a pipe or tank, this information can be very helpful in determining how fast the spilling chemical can be expected to leak and travel.
Concentration and Dose Location Calculations
ALOHA also allows the user to make Concentration and Dose Location calculations. This function allows the responder to determine what the level of a spilled chemical will likely be at a particular location downwind from the spill. This will help decide on strategies about what equipment to wear at different locations during response, and even for determining shelter-in-place strategies. The user will have to enter the data into ALOHA regarding the distance downwind and the crosswind distance, which is the perpendicular distance from the downwind axis.
ALOHA doesn't incorporate the effects of particulates, fires or chemical reactions, or chemical solutions or mixtures. The use of ALOHA should be avoided in these instances except in certain situations. The ALOHA software allows the user to check the ALOHA Selection Key to learn about these exceptional cases.
ALOHA can't help with indoor releases, during rain or snow, for distances over six miles from the release point, for releases lasting over an hour, with hilly terrain, or with the "canyon" effect of urban areas with high-rise buildings. The tendency of large buildings and topographical conditions like hills to cause the spilled plume of chemicals to not move in a straight line is named "terrain steering."
If the chemical is not in ALOHA's chemical library, or if there is no known LOC (Level of Concern) for a chemical, ALOHA cannot produce a "footprint." LOC itself is a debated item. It represents an educated guess at the concentration of a chemical in the ambient air that the public could be exposed to without lasting health problems. LOC does not represent the lowest value for these chemicals.
For more information, go to the NOAA website response.restoration.noaa.gov and look for the Level of Concern page, and other ALOHA information.
ALOHA is revised periodically. ALOHA is used in conjunction with MARPLOT, a comprehensive mapping program developed by the United States Environmental Protection Agency and the U.S. Department of Commerce, National Oceanic and Atmospheric Administration, Bureau of the Census. The plume modeled by ALOHA can be plotted out on a MARPLOT map to help determine vulnerable areas and facilities.
ALOHA, MARPLOT, and other CAMEO programs and software can be used in conjunction with the BOLDER software to provide the ultimate responders' tool. The BOLDER software does not include ALOHA or other off-site consequence analysis software programs, so to determine the likely spill scenario that must be entered into the BOLDER software, a facility, responder, or planning agency must make these off-site consequence analysis determinations first. ALOHA, ARCHIE, and any other number of modeling software may be used to accomplish this. Calculations can also be made by hand using equations and methodology, such as is included in HARM. The Hazards Analysis Resource Manual (HARM) was produced by the Office of Environmental Technology at Arizona State University-East with the aid of USEPA, the Federal Emergency Management Agency (FEMA), and the Arizona Emergency Response Commission (AZSERC). For more information about getting a copy of HARM, contact Daniel Roe, Executive Director of the AZSERC at (602) 231-6345, or at 5636 East McDowell Road, Phoenix, AZ 85008.
ALOHA Information on the Web
NOAA OR&R website: response.restoration.noaa.gov (with Ask Dr. ALOHA" archive, Level of Concern page, and other ALOHA information)
NSC web site www.nsc.org/ehc/cameo.htm (with update files to download, frequently asked questions [FAQs], users' forum, CAMEO Today archive)
You can also view or download the ALOHA Decision Keys from http://response.restoration.noaa.gov/cameo/toolkit.html This can be helpful for determining worst case scenarios for chemicals with unusual behaviors during spills, like aqueous hydrofluoric acid. Tools for ALOHA instructors can also be found at this site.
More information about ALOHA and CAMEO is available at http://www.epa.gov/ceppo.