" This Syrvey Report and any recommendations made herein are for the specific facility evaluated and may not be universally applicable. Any

recommendations made are not to be considered as final statements of NIOSH policy or of any agency or individual involved. Additional NIOSH Survey Reports are available at http://www.cdc.gov/niosh/surveyreports.

IN-DEPTH SURVEY REPORT OF CONTROL OF RESPIRABLE DUST AND CRYSTALLINE SILICA

FROM GRINDING CONCRETE

at This document was provided by the Battelle Library. 614-424-6302 Messer Coastruction Newport, Kentucky and Baker Concrete Construction Dayton, Ohio



### REPORT WRITTEN 

### BY Alan Echt Stanley Shulman

REPORT DATE August, 2002

REPORT NO EPHB 247 - 21

US DEPARTMENT OF

### REALTII AND HUMAN SERVICES

Public Health Service Centers for Disease Control and Prevention Nauonal Institute for Occupanonal Safety and Health DIVISion of Apphed 

### Research and Technology Engmeenng and Physical Hazards Branch 4676 Columbia Parkway. Mall Stop R-5 CmcmnatI, OhIo 45226-1998SITES SURVEYED Parkmg Garage (New Construction), Newport, KY Performing Arts Center (New Construcuon), Dayton, OH SIC CODE 1771 (Concrete Work) SURVEY DATES February 2 and 13, 2001, and Apnl 12,2001 SURVEYS CONDUCTED BY Alan Echt Wilham A Heitbnnk Stanley A Shulman Daniel S Watkins Ronald L Kovern 

### EMPLOYER REPRESENTATIVE 

### CONTACTED Jay Zmt Messer Construction Floyd Turley Baker Concrete Construction



# "

11DISCLAIMER

Mention of company names or products does not constitute endorsement by the Centers for Disease Control and Prevention

11lABSTRACT

When construction workers use hand-held grinders to smooth poured concrete surfaces 

### after forms arc removed. they nsk overexposure to respirable dust and crystalline silica A previous study at a stadium construction site revealed TWA respirable crystalline silica exposures that ranged from 56 to 830 ug/nr' (the NIOSH REL 1S50 }lglml) and TWA respirable dust concentrations rangmg from 0 38 to 6 9

### mg/rrr' &ilCOSlS ISan occupational respiratory disease caused 

### by mhalmg respirable crystallme silica dust Sihcosrs ssrrreversible, often progressive (even after exposure has ceased). and potentially fatal Exposure to crystalline sihca dust occurs In many occupations, mcludmg eonstructron Because no effective treatment exrsts for sihcosrs, prevention through exposure control is essential The study descnbed here compared the exposures during the use of off-the-shelf local exhaust ventilation shrouds WIth that of no shroud The arm was to quantify the exposure reduction that could be achieved through the usc of shrouds, and to compare shroud effectiveness Two grinders 

### from different manufacturers were studied, at three study sites Two shrouds were evaluated for each type of gnnder for a total of four shrouds The gnnder WIth no control vaned from site to srte, since this was furnished by the employer The same vacuum cleaner was used toprovide exhaust air flow at both SItes An aerosol photometer was paired Wlth an ~KC alummum cyclone to measure respirable dust exposures In the cement mason's breathing zone

f With 95% confidence, the results indicated at least 90% reduction due to the controls for each of the four shrouds at an average flow rate of 122 cfm Results indicate lrttle difference between the reductions m the different grinder/shroud pairings ThIS study demonstrated effective control can be achieved for hand held gnnding of concrete

!

IV-- -- ------ - - --

INTRODUCTION The Nanonal Instuute for Occupational Safety and Health (NJOSH) IS located in the Centers for Disease Control and Prevention (CDC), under the Department of Health and Human Services (DHHS) NIOSH was established in 1970 by the Occupational Safety and Health Act, at the same tune that the Occupational Safety and Health Admnnstranon (OSHA) was established m the Department of Labor (DOL) The aSH Act legislation mandated NIOSH to conduct research and education programs separate from the standard-setting and enforcement funcnons conducted 

### by OSHA. An rmportant area of

### MaSH research deals With methods for controlimg occupational exposure to potential chemical and physical hazards The Engrneermg and Physical Hazards Branch (EPHB) of the Drvision of Applied Research and Technology (DART) has been given the lead within NlaSH to study and develop engineering controls: and assess their Impact on reducing occupational Illness Smce 1976, EPHB (and Its forerunner, the Engmeermg Control Technology Branch) has conducted a large number of studies to evaluate engmeenng control technology based upon industry, process, or control techmque The objective of each of these studies has been to evaluate and document control techniques and to determine the effectiveness of the control techniques In reducmg potential health hazards m an mdustry or for a specific process .' .

»< Tills ISthe report of a project that Involved VISIts to three sites where constructronworkers were USIng hand-held grinders to finish the surface of poured concrete walls by removmg form lmes, smoothing patches, and removing other surface irregularities This study sought to quantify the exposure reduction that could be achieved through the use oflocal exhaust ventilation during gnndmg In this case, the local exhaust ventilation consisted of a vanety of commercially available shrouds or hoods mounted OD the grmder and connected via a flextble corrugated hose to an industnal vacuum cleaner

OCCUPATIONAL EXPOSURE TO CRYSTALLINE SILICA Silicosis IS an occupational respiratory disease caused bymhahng respirable crystalline silica dust Sl1tCOSlS IS irreversible, often progressive (even after exposure has ceased), and potentially fatal Because no effective treatment exrsts 

### fur SIliCOSIS, prevention through exposure 

### control IS essential Exposure to respirable crystalhne silica dust occurs 

### Inmany occupations, including construction Crystalhnesihca refers to a group ofmmerals composed of srhcon and oxygen I Crystalline means the oxygen and silicon atoms are arranged m a repeatmg three-dimensional pattern I Crystallme sihca can be found m many forms The three major forms of crystalline sihca are quartz, cnstohahte, and tndynute I Quartz 1Sthe most common 

### Conn of crystallme sihca 1 Respirable means that the dust particles are small enough to reach the region of the human lung where gas exchange occurs I , ,

When proper practices are not followed or controls are not mamtamed, respirable crystallme sihca exposures can exceed the mOSH Recommended Exposure 

### LImIt (REL), the OSHA

1-----


## _ .. -- - - ------------

Pertmssible Exposure Lumt (PEL) or the Amencan Conference of Governmental Industrial Hygienists (ACGllI) Threshold LImit Value (TLV) 1:-4 The N10SH REL for all forms of respirable crystallme salzca rs a lO-hollr, tune-we1ghted average level of 0 05 rmlhgrarns per CUblC meter (mg/m') NIOSH has classified crystalline silica as a potential occupational carcinogen Therefore, NIOSH recommends that employers make efforts to reduce exposures below the REL



# -- The OSHA PEL for respirable dust contammg 1% quartz or more is expressed as an equation 10 Respirable PEL = (% SIlica) + 

### 2 Thus, Ifthe dust contains no crystalline silica, the PEL IS 5 mg/m', and if the dust IS lOO% crystalline sihca, the PEL IS 0 1 mg/nr' For tndynnte and cnstobahte (two other forms of crystalhne silica), OSHA uses half the value calculated using the formula for quartz The ACGrn TLVs for cnstobahte, quartz, and tndyrrnte are all 005 mg/rrr' 

### ME1'HODS EXPOsure Assessment The extent of the reduction m the breathing zone respirable dust exposure of the construction worker when local exhaust ventrlauon (LEV) was used compared to (he exposure when no LEV was used served as the measure of the effectiveness of the controls evaluated In this study Respirable dust exposure was measured usmg an aerosol photometer (Hazdus1 Il, Envtronmental Devices Corporatron, Haverhrll, MA) WIth arespirable dust pre-selector (SKC alurnrnumcyclone. SKC. Inc. Eighty-four, PA) placed before the sensor A 37-nun diameter 5-lJ.mporesize filter III a two-piece cassette in hne after the sensor was analyzed for crystalline Silica to determine the percent crystalhne srlrca Inthe dust as an aggregate over the samphng penod 

### nus enabled an estrmate of the reduction of'crystalhne silica exposure due to the use of the controls, SInce crystalline sihca exposure could not be measured IIIreal time The aerosol photometer was mollified by the manufacturer. mcludmg the installanon of a new mternal pump to allow samphng at a flow rate of2 5 liters per minute, the now rate required for the pre-selector to meet the ACGlWISO/CEN curve for respirable dust (50% samplmg efficiency (cut-point) =40 11m) The aerosol photometer 

### samples at l-second mtervals and provides results ill terms of concentration (mg/m') The pre-selector/seasor/filter unit was clipped to the employee's lapel, connected via flexible tubing and cable to the unit housing the pump and electronics 

### 'flusunit was worn on a belt around the employee's waist Bulk samples of settled dust were collected and analyzed m order to determme whether substances were present which would interfere with the crystalline sihca analyses Gravunetnc analysis for respirable particulate was earned out

### Wltnthe following modificanons to 

### NIOSH Method 0600 l) The filters and backup pads were stored In an envrronmentally

2 /controlled room (21±3 QC and 50±5% relative hurmdrty) and were subjected to the room

conditions for at least two hours for stabihzation pnor to tare and gross weighing, and, 2) Two wergnmgs of the tare weight and gross weight were performed. S The difference between the average gross we,ght and the average tare weight was the result of the analysis The hmit of detection for thts method was 0 02 mg Crystallme sihca analysis offilter and bulk samples was done using X-ray drffraction NIaSH Method 7500 was used with the following modifications I)Filters were dissolved m tctrahydrofuran rather than being ashed m a furnace, and, 2) Standards and samples were run concurrently and an external cahbranon curve was prepared from the integrated mtensiues rather than usmg the suggested normahzanon procedure j These samples were analyzed far twa forms of crystalhne sihca quartz and cnstobahte The 

### hnuts of detection for quartz and cnstobahte on filters were 0 01 and 0 02 mg, respectively The hnut of quanntation IS 0 03 mg fOTboth quartz and cnstobahte The hmits of detectJ.onIn bulk samples were 08% for quartz and 1% for cnstobalite The hrrnt of quantitauon was 2% for both forms of crystalline SIlica Inbulk samples
Vacuum Cleancrhrfo..rmancs Evaluabon Vacuum cleaner pcrfonnance was measured interms ofthc static pressure andair flQw through the shroud-hose-vacuum cleaner system Static pressure was measured by msertmg a smooth-walled metal tube the same diameter as the vacuum cleaner hose m Ime with the hose between the shroud and the vacuum cleaner and measunng the sucuon USing aU-tube manometer One end of the manometer tubing was placed on a hole dnlled IIIthe SIde of the metal tube while the shroud was held against a flat surface Centerlme al.Tvelocrty was measured at the same point using a rnultr-parameter venulation meter (Velocicalc Plus model 8386, TSI Incorporated, St Paul, MN) Static pressure and centerline aITvelocity measurements were made 

### WIth the vacuum cleaner runnmg and 

### WIth the grinder on and off Air flow was calculated by mulnplymg 0 9 centerlrne velocrty bythe cross sectional area of the hose ~ Pressure and al( velocity measurements were made a sufficient number of hose diameters from the tool and the vacuum cleaner to

### rmugare the effects of turbulence on the measurements Expenmcntal 

### DeSIgn For the tnals where the control was used, four hand-held right-angle gnnders were purchased winch were identical to tWo of the types used by the contractors with whom we worked on this proj oct, two MIlwaukee model 6153-20 11,000 rpm gnnders (Mrlwaukee Electric Tool Corp , Brookfield, WI) and two Metabo model 11025 10,000 rpm gnnders (Metabowerke GmbH, Nurtmgen, Gennany) Four local exhaust vennlanon shrouds were purchased, one of'which was sdenucal to the 

### type used by one of the contractors (Vacuguard, Pearl Abrasive Co. Commerce, CA) The other three were selected from those rdentrfied through a search of product literature and the Internet (Dustcontrol, Transmatic 

### me, Wilmington, NC~ and "full dust shroud" and "cut (edgmg) shroud," Sawtec, Oklahoma City, OK) Those selected appeared to be ruggedly constructed, easily mounted on the tool, and readily available for purchase, The combmanons of !

3Milwaukee 6153~20 MIlwaukee 6153-20 Metabo II 025 

### Metabo 11025

gnnder 

### +Dustcontrol gnnder + Sawtec cut gnnder 

### +Pearl grinder 

### + Sawtec full shroud (edging] shroud Abrasive Co dust shroud Vacuguard shroud Table 1 - Grtnder/Shroud Pairs shrouds and grinders used for the study are listed In Table 1 These gnnderlshroud patrs were connected Via corrugated flexible hose to an mdustnal 

### vacuum cleaner (DC 3700 C, 

### Dustcontrol .AB. Norsborg, Sweden) The manufacturer-reports that this unit has a maximum flow capacity 

### of 320 

### rrr'Ihr (188 cfm), and a maximum negative pressure 

### of 21 kPa (84 tilw g) 

### The grmders were fitted 

### WIth 4-lU diameter diamond cup wheels (PW series, Pearl Abrasive Co •Commerce, CA) For the trials where no control was used, the construction workers used the gnnder and gnndmg wheel 

### normally provided to them bytheir employer

The aim was to estimate the reduction in dust produced 

### by the units With controls compared to that produced by those without controls Percent reduction was estimated by Estimated % Reduction 

## = 100 x [1 ~ (control mean)J( no-control meant] 

### Smce there was mterest 

### m comparing reductions ofshrouds 

### for the same 

### grmd: ngurnt, 

### the randonnzanon was earned out so as to first have randomized palfS of the two gnndertypes, and then randomly order the two shrouds and no control withm each of the gnnder types An example randorrnzancn appears in Table 2, below

a e - An Example Ran ormzanon Grinder Type I Run 'l=Control I Run 2=No-control 

### Run 3 =Control 

### 2

, Grinder Type 2 Run 4=ControJ 2 Run 5=No-control 

### Run 6 =Contror I T bI 2 d

In the above example. Control 1 for grinder 1 was different from control 1 for gnnder 2, and analogously for control 2 The actual design differed from that shown above m that m each set of SlXruns) one no-control nm was removed, 10order to lumt worker exposw.-e to dust (the nocontrol ron that was removed was alternated between the two types of'gnnders) Each of these sets of five runs consnruted-a block Multiple blocks were collected at each SIte Control runs consisted of gnndmg for approximately five minutes, while most no-control runs were restncted to about two and a half rnmutes of gnndmg, begmnmg wrth site 2 (The ann was to reduce worker exposure to the dust) The benefit of tlns design was relatively quick assessment of controls and reduced exposure to the worker, which was only possible because of the availabihty ofreal-tnne mstruments Gnndmg at site 1 consisted of walls and columns m an open area, site 2 involved gnndmg on a wall In an underground parking garage, SIte 3 involved grinding

# on a wall u:;aa comdor open to the outside at one end Srtes 1 and 3 were both new construction at the same performing arts 4--_._- --------- _._.- 

## -_
.._---------

center, while Site 2 was a parkmg garage at an entertamment and shopping complex At site 1 sampling was a much more arduous task than at the other sites because of very low temperatures at that site on that day StatIstIcal Procedures and Handhng...QfData From the sequenhal measurements in each tnal a smgle summary measure was calculated, the geometnc mean of the dust concentrations Example data are given ill Figure 1,10which for both of the condinons shown the measurements and the geometnc mean are plotted The geometric mean computation required calculation of the average dust concentratJ.on on the natural log scale At site 3 there were over 88 dust concentrations equal to zero, all dunng tnals when controls were run In order to take the logarithm of these dust concentrations, 0 01 was added to all zero dust concentrations at that sue, before taking the log Addmon of

### thrs small value made all values positive Since there were almost 5000 measurements at site 3. the inclusion or alteranon of the 88 zero values had small effect on the estrmated geometric means and the estimated comparisons Dunng each tnal, a penod oftime had to elapse before the measurements approached a stable mean An mmal increase in measurements was true for most control-off tnals' For example, the "no control" figure m lhe FIgure I ~ata mdicates that 

### It took over 20 dust concentration measurements ( at one second tntervals) to stabihze The staustical model was fitted for several versions of the data However, smce results differed httle from the unaltered data and It was difficult to determine the optimal number of values to remove, results are presented for the geometric means calculated for all the data collected dunng the tnal The geometnc means were transformed to the natural log scale and used as the response III a statistical model The two SItes studied here were thought to be too few to be regarded as a random sample from a larger population Thus, the results 

### apply only to the srtes Studied

### here, though. there lS no reason to tlnnk that the results are not representatrve of those that would be obtamed at smular sites In the stansncal model individual means were allowed for each SIte, as well as for the VariOUS control-shroud combmauons at each site The blocks were treated as a random sample, as was the ordermg within each block The data appear to meet the assumptions of approXlmate normahty The model provided estimates of the reduction due to the controls, and also confidence hrmts for the nummum reduction, based on the Student's tdistnbunon One-sided confidence limits were provided SlO~ reduction due to the control was expected Correction was made for multiple comparisons VIa the Bonferrom method, since many confidence intervals were based on the same data 1 AJI confidence statements gIVen below hold with at least 95% confidence 

# . ,

5 

### • ,RESULTS

Effect of Controls on Resprrable Dust Exposuref> The data In Table 2 indicate that the levels observed were quite vanable among the three sites SIte 2 gave the highest dust concentranon measurements most no-control 

### dust concentration measurements were greater than about 40 mg/m' end control dust concentranon measurements were greater than 0 6 rng/m' Site 3 differed from site l in that the control dust concentranon measurements were all lower than about 0.3 mg/rrr' at site 3, and all greater than about 08 mg/m' 

### at sue l,even though the no-control values overlapped The average srte 2 and SIte 3 estimated control reductions were all between 98 and 99%, and none of the reductions at the two SItes for each of the gnnder types differed in tests at the 5% level Thus, the differences between site 2 and 3 appear to be due to higher control geometric means at site 2 than at site 3 As was explained In the previous section, the temperatures durmg the srte 1VISItwere 

### very- low, and tlus may have affected the results 

### By the statistical tests, site 1 results indicated lower reductions than those for the two other site visits for all four units, based on tests at the 5% sigmficance level Even With the SIte 1 results included In the overall averages, reductions were quite high about 97% for thetwo Metabo units, and about 96% for the Milwaukee ~ts 95% lower confidence hrmts for each grinder, averaged over all three sire VISIts, exceeded 92% reductron (see FIgure 2) Based on percent reduction, we can say that all four grinders were SImilarly effective, and all reduced the generated dust by at least 90% relauve to the no-control, when averaged over all three sstes ThIS conclusion holds with 95% confidence Crystalhne SIlIca Samplmg Results Analyses ofthe respirable particulate sample collected on a filter using the aerosol photometer at SIte 1 revealed that the sampled dust contained 68% quartz by werght Two samples of this type were collected at site 2, one In the mommg and one In the afternoon The morrung sample contained 7 3% quartz by weight, while the afternoon sample contained 10 2% quartz byweight At site 3, a single filter sample was collected, and was found to contain 8 6o/lt quartz by weight The mass of quartz found on that filter, 0 03 mg, was between the lumt of detection and the lurnt of quanntanon, and should be regarded as a trace value No cnstobahte was detected to any of the samples

- Vacuum Cleaner Perfonnance EvaluatIon ( The average air flow rate for the MIlwaukee gnnder and Sawtec shroud was 144 cfm (n=4, range 129-151 cfin) The static pressure for thrs pamng was 18 inches of water (m w g) The average all flow rate for the Milwaukee gnnder and Dustcontrol shroud was 117 din (n=e, range 109- 121 cfm) The static pressure for tlus pamng was 32 10 w g . The.average air flow rate for the Metabo gnnder/Sawtec shroud parr was 130 

### cfm (n=3, range 108-130 cfm), wlnle the average air

6 

## .. •·,

flow rate [or the Metabo gnnder and Pear} shroud 

### was 105 cfm (n=J, range 95-113 cfin) The

static pressures measured for those gnnder shroud cornbmatrons were 20 In Wg and 28 m w g , respectively ConclUSIOns and RecommendatIons 

### ThIS study showed that all four shrouds were equally effective, and that

# an reduced dust exposure by at least 90% Thus, the choice of shrouds can be made based upon the preferences of the cement finisher and their employer They should work together to select the shroud best suited to the task Anecdotal mformanon conveyed bythe cement finishers Involved Inthis study indicate that issues such as hght weight, ease of use, abihty to see the work surface, and durability were all Important factors In determnung shroud preference References 1 Bureau of Mines [1992] Crystalhne srhca 

### pnmcr Washmgton, DC US Department of the Interior, Bureau of Mines, Branch of Industrial Minerals, Spectal Pubhcation



# . 2 MOSH [1997] NlaSH pocket guide to chemical hazards CmcmnatJ/OH 

### US Department of Health and Hurnan Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Oceupational Safety and Health. DHHS (NIOSH) Pubhcation No 97-140 

### 3 29 CFR 1910 1000 [2001J Occupational Safetyand Health Admrmstranon air contaminants 4 ACGTH [2001] Threshold hrmt values for chemical substances 

### Cmcmnati.Dfi Amencan Conference of Governmental Industnal Hygienists 

### 5 NJOSH [1994]
T mOSH 

### manual ofanalynca; 

### methods 4th rev ed ,Eller PM, ed Cmcmnan, OH U S Department of Health and Human Services, Public Health Service, Centers forDISease Control and Prevention, 

### National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No 94-113
6 

### ACGrn [2001] 

### Industrial venulanon, amanual 

### ofrecommended practice 24lh ed Cmcmnau, OH- Amencan Conference of Govemrnental Industnai Hygrerusts Pg 94 7 MIller, Jr, R G (1981) Simultaneous Statistical Inference, 2nd Edinon Springer-Verlag, New York, p 8

7Table 2

Estimated Geometric Means and %Reduchons due to Controls for Tnals from the Three SItes

SIte Numbl:l' Block No Mew.bo Me1l\bo MIlwaukee MII•.••.aukee Control" Pearl SDWtec full dml Dustcontrol Sawte~(:111 (ed!;lag)

GM(% Rc:d) sllroud GM(% Red) stm:IUC

GM(% R..d) GM(%Red)

~ I I 12 31~ I 16(91) 078(94)

941· 2 17(77) 071(92) 1 2 457 A A 1 67(63) 

### o 8.6(81} 2 3 7034 

### o 74(99) 077(99) I 15(98) 036(99) 2 4 )q 55 167(%) {)97(98) 1 10(97) 

### o 94(98} 2 5 4828 152(97) 1 14(98) r 21(98) ] 42(!l7) 2 6 84 11 l58(98) 1 73(98) 1 67(98) 30l(%} 2 7 485D 1 18(76)" • {t 69(86)° 060(88)D I 32(7~)]) 2 8 4649 I 12(98) !I B9(98) ] 19(97) I 05(98)

+ 3 9 ] 65 0061(96) 

### o (9)(94) o I I (93) 

### o 13{!)2) 3 

## to 25] 0048(98) 0095(96) 

### o 027(99} 0031(99) 3 Ll 915 0064(99) 0067(99) 0092(99) 0016(98) 3 12 2445 

### o 11(995) A 03[(99) 

### o 24(99) AData not collected

+ Two control-off tnal were earned out ill block 1, one WIth the Metabo and one WIth the Milwaukee gnnder CA different no conrrol gnnder was used at each site For the control settmgs, the same umt (gnnder plus shroud) was used at each Site o Value treated as possible outlier and excluded from data analyses The low %reducuons are evidence that the no control measurement IS an outher

{

8Figure 1 Example Data

Figures below provide the actual data for two runs, plus the geometnc mean for each tun Ln(Geometnc mean) was used In 

### stansncal analyses Note that the scale for "no control" differs from scale for the control urut

Sa wiec cui (edging) shroud

6 -~-- '-- -- .. ----

2. ---- .. - -- --



# -- . ~5!J

:ilJO 

## t 

# •• 150

100

50 - No control

5 ~---------.--

3-· 4 - ----~--~~_l

Seconds htm start 01'Insl t----~.-----.:!!.4-- .---.

~

O+---~. 

# 1

. = ~ ~ i ~ ~ 2 ~ : 5 ~.!~ 1 ~ •

Secon(i!> rom start of tnal

_____ .__ . --.J Figure 2 Estimated Reductions, SImultaneous l-sided 95% Confidence Limns By SIte Sawtec Sawtec full Dustcontrol cut 100% ..,---=---=-_:.; 90% 80% ~ 70% 

### 5' 60% 

# g 50% ~ 40% 0:::: 30% 20% 1Q% 0% Pearl Average All Sites

r.» OJ (1'1 r.n (f.I (tJ (II Cfl r.J'J ~ (J) c.o 

# Z ~ 0 en i- S- :1 ffi" is" 

### ..•. ii" i; ~ it ii i: c 

# i

(II GO

~ ~ ...• I>.J W ....• N w ...•. N W ..•. I\l ~ ::::L 

# l!:lower C0nfid~nc::eLI~lt • Estimated Redl,l;llon] 0 :::J (1

~ 

## a
~

9 I