Calculations And Occupational Exposure Limits

Calculations and OccupationalExposure LimitsDr. Peter Bellin, PhD, CIHEOH 466AThe Occupational EnvironmentDefinitions Vapors Gases Aerosols– Particulates: dust, fume, fibers, nanoparticles, smoke– Mist Particle size considerations– Inspirable Particulate Mass (IPM)– Thoracic Particulate Mass (TPM)– Respiratory Particulate Mass (RPM)2Calculations Evaluation Control1

OELs3OELs Concentration– Parts per million Maximum vapor concentration VPl / VPatm x 106– Milligrams per cubic meter Normal temperature and pressure– 760 mm Hg and 25 C– Molar volume 24.5 Liters per mole4Calculations Evaluation Control2

OELs Time-weighted average (TWA)Ceiling value (C)Short-Term Exposure Limit (STEL)Immediately Dangerous to Life and Health(IDLH)5OELs Exposure limits for gases and vapors areestablished in terms of ppm mg/M3 values are determined bycalculation, conversion based upon anassumption of NTP If samples are taken at P and T conditionsvery different from NTP and results are inmg/M3, results must be corrected.6Calculations Evaluation Control3

OELs Conversion between ppm and mg/M3mgl 24.453moleY ppm MgMWmolegY ppm MWmgmole X3lM24.45moleX7OELs Time weightedaverageconcentration ismeasured by takingone or moremeasurements ofconcentration over awork shift.nTWA C Ti 1ni i Ti 1i8Calculations Evaluation Control4

OELs 8-hour TWA: average exposure over an eighthour time period (normal work shift) 8 hour TWA ni 1CiTi8 hours9OELs Example: A press cleaner is monitored forexposure to ethanol. The data tion(hours)14101.522503.5375210Calculations Evaluation Control5

OELs Sample TWA calculation410 ppm 1.5hrs 250 ppm 3.5hrs 75 ppm 2hrs1.5hrs 3.5hrs 2hrs1640 ppm hrs 234 ppm7 hrsTWA 8 hr TWA 1640 ppm hrs 205 ppm8hrs11OELs Short Term Exposure Limit (STEL) The concentration to which workers canbe exposed continuously for a short periodof time without suffering:– Irritation– Chronic or irreversible tissue damage– Narcosis of sufficient degree to increase thelikelihood of accidental injury, impaired selfrescue or materially reduce work efficiency12Calculations Evaluation Control6

OELs Short Term Exposure Limits– Usually a 15-minute period– Should not be exceeded anytime during aworkday, even if the 8-hour TWA is below theOEL. (8-hour TWA OEL will be a lowerconcentration)13OELs Short Term Exposure Limits– Exposures above 8-hour OEL but belowSTEL Should not be longer than 15 minutes Should not occur more than 4 times per day There should be at least 60 minutes betweenexposures in this range. Example: diethylamine, TLV:– 8-hour is 5 ppm.– STEL is 15 ppm.14Calculations Evaluation Control7

OELs Ceiling Value Concentration that should not beexceeded during any part of the work day. Designated by a “C” preceding substancelisting.– Example, Acetaldehyde, STEL 25 ppm, andhas a ‘C’ designation.15OELs Mixtures If the biological effects of a group ofchemicals are independent, compare eachexposure to the OEL. If the ratio: 8-hour TWA / OEL is– 1 exposure is below OEL– 1 exposure is above OEL– Do this for each chemical independently16Calculations Evaluation Control8

OELs Additive effects– Similar toxic effects– sum the ratios of 8-hour TWA / OEL– K sum of these ratios– If K 1, combined exposure is below OEL– K 1, combined exposure is above OEL17OELs Adjusting OELs to different work shifts Allowed exposure should be changed toaccount for duration of exposure OSHA model Brief and Scala Model18Calculations Evaluation Control9

OELs OSHA Model (T 8 hours)TWA′ PEL 8 hrsT hrs19OELs Brief and Scala Model (T is shift in hours)8 hrs 24 T hrsTLV′ TLVT hrs16 hrs20Calculations Evaluation Control10

OELs Example 1,2 trichloroethane (a solvent) has abiological half life of 16 hours. What modified PEL or TLV would beappropriate for people who work 3 12-hourshifts per week exposed to thecompound? TLV and PEL are both 10 ppm21OELs SolutionOSHA MODELPEL′ 8 hrs 10ppm 6.7ppm12 hrsBrief and Scala ModelTLV′ 8 hrs 24 12 hrs 10 5ppm12 hrs16 hrs22Calculations Evaluation Control11

Evaluation and ControlDr. Peter Bellin, PhD, CIHEOH 466AThe Occupational EnvironmentPre Inspection Research Before you visit, research the process.Learn some terms before visit.Look for records of previous inspections.Become aware of hazards you mightexpect to see.24Calculations Evaluation Control12

Initial Walk Through Observe work practices and environmental conditions.Look for evidence of potential safety and health hazards:dust, grime in air or on surfaces. Other signs? Observe operations: cutting, heating, mixing, bagging. Observe controls: engineering, administrative, personalprotective equipment. Interview Workers: often have important and relevantinformation Make flow diagrams, notes, take photos if possible.25Basic Elements: Qualitative IHSurvey List locations covered by the survey. The elements of this list willdepend on the size of the facility being evaluated as well as the level ofevaluation.Description of operations. For each worksite (location) listed, list theoperations conducted. This description should include some mention of theoperations that may generate hazards.List of hazardous materials. A list of materials used at each operationshould also be listed. This list should be based on the material content, notthe manufacturer: so if several suppliers of acetone are used, they can becounted together.List of hazardous physical agents. Sources of heat, noise, nonionizingradiation (microwaves), ionizing radiation, ergonomic hazards noted shouldbe listed.Existing controls. Describe ventilation used, personal protection worn andadministrative controls in place.Personnel information. Number and job titles of personnel working in thearea should be collected. Comments from interviewed workers should beorganized.26Calculations Evaluation Control13

Quantitative IH Survey OSHA Inspection Exposure characterization Statistically reliable evaluation requiresmany measurements Sampling strategy is needed Design of survey will depend on aqualitative IH survey27Quantitative IH Survey: How areMeasurements Taken? Integrated over time: minutes to hoursGrab samplingSize-selective samplingDirect reading instrumentsColorimetric tubesSampling mediaOSHA or NIOSH methodsAccredited laboratory for analysis28Calculations Evaluation Control14

Quantitative IH Survey: RecordKeeping Minimum data elements– Plant, location, date, worker ID, job titles,process name, time on/off, inspector name– Document calibration and sample handling– Provide a sound basis for future reference(legal proceedings?) Notify affected workers of monitoringresults29Initial Design of Control Design Stage: Plan new construction,systems with worker protection in mind. Also consider:– air pollution– water pollution– waste minimization– hazardous waste control– accident/disaster control30Calculations Evaluation Control15

Industrial Hygiene Control Once a system is constructed, methods tofollow in correcting a problem. Priority of control– substitution, process change– ventilation/engineering– administrative– personal protection31Dilution Ventilation– Also known as General Exhaust Ventilation substances of low toxicity. contaminant source large or diffuse. prevent buildup of explosive concentrations in astorage area. applies to gaseous hazards, not particulates. when local exhaust ventilation is not feasible. costs of clean air (make up air) not prohibitive.32Calculations Evaluation Control16

Local exhaust ventilation Preferred with:– substances of high toxicity– unpredictable or sporadic generation– point sources– aerosols– prevent pollution of air, water, etc.33Dilution Ventilation Theory Dilution ventilation may be applied insituations where vapor concentrationbuild-up and decay can be predicted. Assumptions when applying equations:– perfect mixing in work area– constant generation rate– clean dilution air is used– no other sources of product in air– no other removal mechanisms than dilution34Calculations Evaluation Control17

General Equation to PredictConcentration Q ( t 2 t1 ) 1 C2 G - (G - QC1 )e V Q –––––Q Volumetric flow of dilution airV the room volumet timeC concentrationG generation rate of vapor35Special Cases C1 0; t1 0; G 0 t G C2 1 e V Q Q36Calculations Evaluation Control18

Special Cases If a long time has passed, then exponentdrops out (long is more than 3 airchanges, Qt / V 3) Cmax G/Q37Special Cases Concentration decay; no more generation.C2 C1e-Q( t 2 t1 )V38Calculations Evaluation Control19

Special Cases How much ventilation is needed to reduceconcentration to safe levels?C1 - C 2Q2 Q1C239Special Cases Volume of air needed to make sure solventconcentration is kept below TLV.Generation rate is known. K is mixing factor.lbs 106 KminCFM MW OELg0.0244 106 K3secM secMW OEL387 40Calculations Evaluation Control20

General Exhaust Ventilation Relative to Local Exhaust Ventilation,dilution (general) ventilation is usually lesssatisfactory.– exposure is spread around a workplace– more air is usually needed to operate asystem– cleaning the air is a problem41Local Exhaust Ventilation Consists of– hood– ductwork– air cleaner– air mover (fans)– exhaust stack42Calculations Evaluation Control21

Hoods Enclosing hoods– Completely enclosing hoods: glove boxesprimary example.– Problem: lack of access to the operation;application may be limited.43Glove Box44Calculations Evaluation Control22

Enclosing Hoods: Booths Booths, with one open side for access.Examples include spray paint booths,laboratory hoods. Allows access to part. Problem: material may escape at theopening.45Laboratory Fume Hood46Calculations Evaluation Control23

Enclosing Hoods: Tunnels Two open faces. Example is conveyerventilation, drying oven. Advantage is toallow parts/material to be passed throughbooth on construction line; amenable toautomated processes. Problem: two open faces allowing escapeof toxic substances.47Exterior Hoods Capture vapors or aerosols emitted by aprocess, using air flow Receiving hoods: designed to capturematerials that are 'thrown' to them Canopy hoods rely on rising air currents, usuallyheated air, to capture material. exhaust flow mustbe large enough to capture rising air. Grinding wheel hoods rely on particle momentumto capture particles thrown off with highmomentum.48Calculations Evaluation Control24

Exterior Hoods49Design considerations more enclosure less cross draft. Capture velocity: the air velocity needed totransport contaminant from point of generationinto the hood. Face velocity: air velocity at the face of the hood(hood opening) Slot velocity: air velocity at slot openings in slottype hoods. Slots help distribute airflow acrossthe face of the hood. Plenum velocity: air velocity in plenum (body ofthe hood), behind face of hood.50Calculations Evaluation Control25

Design Considerations Required capture velocity can be calculated,and depends on hood design and airflowinto the hood.– Round or square openings: Velocity drops withsquare of distance from hood opening.Q( no flange)210x AQV ( flange)0.75(10x 2 A )V 51Design Considerations Round or squareopenings– Bell mouth inlet52Calculations Evaluation Control26

Design Considerations– Canopy hood design incorporates dimensions ofthe hood and height of the hood above operation: V Q / (1.4 x Perimeter x H )53Design Considerations– Slot hoods are used to distribute airflow along awide surface, such as a tankQ3.7LxQV 2.8LxV (no flange)( flanged )54Calculations Evaluation Control27

Design Considerations Slot hood55Ductwork Must carry air and aerosol from the pointof generation (capture) to air cleaningsystem. Terms– Static pressure: negative (or positive pressurecreated by a fan, drives the air flow in thesystem.– Velocity pressure: Pressure of moving air: thiscan be measured and is used to measure airvelocity.56Calculations Evaluation Control28