Calculating toxic impacts to indoor populations/media/Documents/Subject Groups...CALCULATING TOXIC IMPACTS TO INDOOR POPULATIONS Anthony G. Sarrack Baker Engineering and Risk Consultants,

CALCULATING TOXIC IMPACTS TO INDOOR POPULATIONS

Anthony G. Sarrack

Baker Engineering and Risk Consultants, Inc., 3330 Oakwell Court, San Antonio, TX 78218-3024 [email protected]

Baker Engineering and Risk Consultants, Inc. has performed quantitative risk analyses (QRAs) for

many facilities including companies handling acutely toxic materials. This paper compares tra-

ditional methods of estimating toxic risk exposure to personnel located within buildings via

current methods and explains the benefits of the updated methods.

1. TRADITIONAL TOXIC IMPACT

CALCULATIONSHistorically, toxic impacts have been assessed simplisticallyby developing geographic contours for threshold values andapplying corresponding vulnerability values for personnelwithin those areas (e.g., a dose function is applied by assum-ing an exposure time). Building occupants were treated in asimilar manner as outdoor personnel, although mitigationfactors were often applied to credit toxic gas detection, ven-tilation isolation, and protective equipment such as escapepacks. Figure 1 shows a typical toxic dispersion predictionalong with several buildings within the toxic plume.

A typical method of predicting consequences for thistype of toxic exposure is to treat indoor concentration as if itwere the same as outdoors and apply a probit equation to theassumed exposure duration for the category of concen-tration. In the example shown in Figure 1, Building 1 is inthe “low” consequence (probability of death) plume whileBuildings 2 and 3 are in the “medium” consequence plume.No buildings are in the “high” consequence plume in thisexample.

Consequences may be reported for occupants byassuming they evacuate the building, even if they wouldactually shelter-in-place. This is not a very good represen-tation of what really occurs. People who shelter-in-placeshould be assumed to remain in the building and continuebeing exposed by the hazard for as long as the hazard ispresent (function of isolation time and blowdown duration).People who evacuate the building should be assessed byaccounting for the predicted concentration profile as afunction of location and determining concentration as afunction of time based on the escape route and evacuationrate. This paper is limited to the impact of personnel shelter-ing in place.

Figure 2 reproduces the scenario depicted in Figure 1,but the labels have been changed to show the specific toxicconcentrations calculated for each of the buildings as well asthe consequence categories that may be assessed for thosebuildings.

Categorizing by toxic concentration can lead to dra-matic discrepancies between predicted severity and actualseverity of a given hazard. In Figure 2, Buildings 2 and 3are both categorized as medium consequence (assigned10% vulnerability), but the concentration at Building 3 issignificantly higher than at Building 2. The vulnerabilityvalues also do not account for HVAC isolation reliabilityor building leak tightness.

2. ENHANCED TOXIC IMPACT CALCULATIONSUsing a detailed method of evaluating toxic risks to person-nel in buildings means accounting for the specific concen-tration predicted at the building for each scenario ratherthan the concentration category. It also requires buildingspecific information such as ventilation intake rate, venti-lation isolation reliability, and building leak tightness tobe factored into calculations. Scenario specific informationsuch as wind speed, direction, and release duration are allrequired to properly characterize the indoor concentrationand resulting vulnerability to building occupants. Theresulting indoor concentration versus time profile alongwith material toxicity forms the basis for defensible and rea-listic occupant vulnerability (OV) calculations. As appli-cable, mitigation factors can be applied to account forprotective equipment.

To provide an accurate risk result for a given toxicrelease scenario, a range of likely release durations andassociated conditional probabilities should be assessed.For example, a release may be modeled as a 5 minute dur-ation (rapid isolation and limited inventory blowdown)with a 75% conditional probability, 15 minute release(delayed isolation) with a 24% conditional probability,and 60 minute release (long duration) for the remaining1%. See the example event tree in Figure 3.

A building’s ventilation and infiltration character-istics should be explicitly modeled in an enhanced toxicrisk calculation. For example, a building may have 6 airchanges per hour (ACH) of forced ventilation with 10% ofthat flow coming from fresh air (ingression ¼ 0.6 ACH),and tests may show that its air exchange rate due to infiltra-tion is 0.3 ACH with 2.5 m/s winds outside. Toxics may bedetected at HVAC inlets causing alarms that alert occupantsto trip the HVAC system, which have been determined tooccur with 98% reliability (Figure 4).

The predicted indoor concentrations as a function oftime for the 60 minute release case with and withoutHVAC isolation are shown for Building 3 in Figure 5.These concentrations are based on standard perfect mixingcalculations, as shown in the following equation.

Cindoor = Coutdoor · (1 − exp (−AXR · trelease))

– Cindoor is the indoor concentration.– Coutdoor is the plume concentration at the near wall of the

building.

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– AXR is the air change rate of the building based onthe air changes from the HVAC system, infiltration, orboth.

– trelease is the duration of the toxic plume.

The probability of death for a given concentration andexposure time is solved using a probit equation of the form:

Pr = A + B · Ln(t · Cn)

Figure 1. Typical toxic plume prediction

Figure 2. Toxic concentrations, concentration categories, and occupant vulnerabilities


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– Pr is the probit value for the toxic dose.– A, B, and n are constants developed by research groups

for a given toxin.– t is the duration of exposure.– C is the concentration of the toxin.

The probit value is converted to a probability of deathusing the probit dose response equation:

OV = 0.5 · 1 + Pr(Cindoor) − 5

|Pr(Cindoor) − 5| erf|Pr(Cindoor) − 5|��

2√

( )( )

Because indoor concentration changes with time, thevulnerability for each infinitely small time step is assessed

Figure 3. Simple event tree of example conditional

probabilities for release duration

Figure 4. Event tree of example conditional probabilities

Figure 5. Building 3 indoor concentration vs. time


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and they are summed, as defined by the following integrateddose equation:

Pr = A + B · Ln

∫t

0

C(t)ndt

( )

In this equation, the variables are all the same asdefined earlier. This integral equation is coupled with thepredicted indoor concentration (perfect mixing) equationto calculate vulnerability for the scenario.

The OV is assessed with the HVAC isolated andagain with it running, and the weighted average OV isapplied based on the reliability of isolating the HVACsystem.

CSIP−tox = [(OVSIP-HVAC-iso × PSIP-HVAC-iso)

+ (OVSIP-HVAC-uniso × PSIP-HVAC-uniso)]× PopSIP × MFSIP-tox

CSIP-tox is the weighted average consequence (number offatalities).

OVSIP-HVAC-iso is the occupant vulnerability calculated forthe SIP with HVAC successfully isolated.

PSIP-HVAC-iso is the probability that HVAC is successfullyisolated.

OVSIP-HVAC-uniso is the occupant vulnerability calculated forthe SIP with HVAC running.

PSIP-HVAC-uniso is the probability that HVAC isolation fails(continues running).

PopSIP is the number of people present in the SIP.MFSIP-to is the mitigation factor for the SIP (applicable if

PPE and indoor toxic monitoring is provided, and gui-dance is provided to evacuate at some threshold concen-tration).

Consequences of being exposed to these concentra-tion profiles as well as other plume durations (same shapes

but end earlier on the graph) are calculated by integratingthe corresponding probit equation for the chemical. Thescenario is split into multiple possible cases for theenhanced calculation (multiple durations, HVAC on oroff), and each case is assessed for likelihood and conse-quence. Results are added to give a better picture of toxicrisk to building occupants.

3. COMPARISON OF RESULTSResults of an enhanced toxic risk exposure calculation canbe significantly higher or lower than results from a simpli-fied analysis. However, results from the enhanced methodare more defensible and should be more accurate. Table 1provides a comparison of results of the example casedescribed above, based on a standard simplified toxic riskcalculation method and the enhanced method describedabove. Results are shown for two leak tightness values(0.3 and 0.1 air changes per hour) and a range of ventilationisolation reliability values (50%, 95%, and 99%). Resultsare presented in terms of the predicted vulnerability forthree release durations (1, 10, and 60 minutes). Blankcells in the table indicate that vulnerability is negligiblefor that configuration.

Notice that the traditional (simple) method is gener-ally conservative, although it can also be non-conservative.In the example above, the reliability of timely sourceisolation is set high (95% success at isolating within10 minutes), so the average vulnerability values (themerged cells to the right of the individual time slot cells)are all less than the simple method. However, if timelysource isolation were less reliable, higher vulnerabilityvalues would be calculated. Cases in which the building isat the high end of the concentration category and therelease lasts long (last row of the table) vulnerabilityresults are predicted to be relatively high (19%–81%).

Table 1. Comparison of results from simplified and enhanced methods


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By implementing the enhanced calculation method,indoor toxic risks are more accurately predicted. Thismeans attention is better focused on areas of true concern.In addition, the method provides a robust way of quantifyingthe potential safety benefit of implementing risk mitigationstrategies such as more reliable toxic gas detection andtimely HVAC isolation capabilities, better leak tightnessof the building, more rapid detection and isolation of thetoxic source, etc. This results in better input to decisionsregarding safety improvements and project priorities.

Following is a summary of significant advantagesgained by performing enhanced indoor toxic risk calcu-lations instead of following traditional calculation methods:

– Enhanced calculations account for release duration andwind speed, which are typically not explicitly addressedin traditional indoor toxic impact calculations. Thesekey parameters have a significant impact on indoor con-centration and are required to accurately predict conse-quences and risk.

– Enhanced calculations explicitly factor HVAC ingres-sion rate, HVAC isolation reliability, and building leaktightness into risk calculations. Traditional calculationstypically use unsubstantiated factors to account forthese parameters or may not address them at all. Byexplicitly calculating the effect of these parameters onrisk results, sensitivity studies can be performed.

– Enhanced calculations use the predicted concentrationat the building rather than the concentration category asinput to consequence calculations. This refinement pro-duces more accurate results and generally reduces con-servatism.

4. CASE STUDY – EXAMPLE OF PRACTICAL

RESULTSResults of a risk analysis that uses the enhanced toxic riskcalculation methods described in this paper can be used toanswer the question, “How safe is safe enough?” Specifi-cally, the results can explain the correlation between build-ing leak tightness and risk, so practical, technicallydefensible decisions can be made regarding the design ofa building that will be used for sheltering in place duringan accidental release of toxic material. The followingexample is provided to clarify this concept.

An onsite building that will be used for sheltering inplace has a ventilation system that draws 0.6 air changesper hour (ACH) of outside air into it. The system is isolatedmanually by occupants based on annual training theyreceive and toxic gas alarms initiated by control room oper-

ators. A review of procedures and interviews with operatorsand building occupants leads to the conclusion that venti-lation would be isolated in a timely manner approximately95% of the time. Its infiltration rate is 0.5 ACH based ona test performed with 2.5 m/s winds outdoors. Theseinputs are summarized in Table 2.

Based on discussions with operators, conditionalprobabilities for isolation times are 75% for 5 minutes,24% for 15 minutes, and the other releases (1%) areassumed to last 60 minutes. Source isolation reliabilityinputs are summarized in Table 3.

Based on these inputs, a QRA is performed and toxicrisk is determined to be 2.8 × 1023 fatalities/year. Todetermine the safety benefit afforded by improving buildingleak tightness, a sensitivity study is performed by assessingrisk with a range of leak tightness values applied. Resultsare summarized in Table 4 and Figure 6.

These results lead to some obvious conclusions, andthey can be used as input to decisions regarding risk mitiga-tion decisions. Following are examples of conclusions thatcan be drawn from this sensitivity study:

– If the building is very leaky (such as 1 ACH), occupantsare better off evacuating than attempting to shelter inplace. This is because evacuating the building limitsthe time a person is exposed whereas sheltering in placeallows the exposure to continue until the toxic cloud hascleared and the building atmosphere has been purged.

– Risk drops significantly as a function of leak tightnessuntil approximately 0.1 or 0.05 ACH. There is littlerisk mitigation afforded by making the building tighterthan this. Therefore the goal value should be in therange of 0.05 ACH.

Because there are many ways to mitigate risk, andeach may affect these results, it is not always straightfor-ward to select the optimum set of changes to most efficientlymitigate risk. For example, in the case evaluated above, thefacility may add toxic gas monitoring within the buiding andprovide training and escape packs. Directions may be given

Table 2. Shelter in place example building input data

Building

AXR

HVAC Isolation

ReliabilityIngression Infiltration

Example SIP 0.6 ACH 0.5 ACH@5mph 95%

Table 3. Source isolation reliability and timing data

Isolation Time

(Minutes) Probability(%)

5 75

15 24

60 1


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to don the escape packs and evacuate the building to a safelocation if toxic gas concentration reaches a specific levelinside of the building. Implementing such a mitigationplan would dramatically lower the baseline risk valuein the building and therefore would also dramaticallyreduce the safety benefit available through improved leaktightness.

5. CONCLUSIONIndoor toxic impact calculations have traditionally been per-formed using simplified methods that fail to provide ameans to quantify the safety benefit of implementingtypical risk mitigation strategies. Results of traditional

toxic risk calculations tend to give conservative resultsthat can cause facilities to designate disproportionateresources to mitigating toxic hazards than hazards that aremodelled more accurately. In some cases traditional calcu-lation methods can also understate toxic risks.

Enhanced toxic risk calculations provide a means ofaccurately quantifying toxic risk to building occupants.They also enable sensitivity studies to be performed toquantify safety benefits available through implementationof potential mitigation strategies. By accurately assessingtoxic risks to building occupants and evaluating safetybenefits available through implementation of potential miti-gation strategies, management can properly select and prior-itize projects to optimize safety.

Figure 6. Risk as a function of building tightness

Table 4. Building toxic risk as a function of leak tightness

Reduction

Configuration ACH Risk Abs %

Corporate EHS Building 4.0E-3

Corporate EHS Building 1ACH 1 4.2E-3 22.7E-4 26.7

Corporate EHS Building 0.75ACH 0.75 3.6E-3 3.4E-4 8.7













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