" 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 45226-1998

This Survey 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. 
SITES 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

"

11
DISCLAIMER

Mention of company names or products does not constitute endorsement by the Centers for

Disease Control and Prevention

11l
ABSTRACT

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 1S 50 }lglml) and TWA respirable dust

concentrations rangmg from 0 38 to 6 9 mg/rrr' &ilCOSlS IS an occupational respiratory disease

caused by mhalmg respirable crystallme silica dust Sihcosrs ss rrreversible, 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

to provide 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 by mhahng 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 In many 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 1S the 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 a respirable dust pre-selector (SKC alurnrnum

cyclone. 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 In the 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 'flus unit

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 Wltn the 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 fOT both quartz

and cnstobahte The hmits of detectJ.on In bulk samples were 08% for quartz and 1% for

cnstobalite The hrrnt of quantitauon was 2% for both forms of crystalline SIlica In bulk samples

Vacuum Cleancrhrfo..rmancs Evaluabon

Vacuum cleaner pcrfonnance was measured interms ofthc static pressure and air 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 III the 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 aIT velocity 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 by the 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

!

3
Milwaukee 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 til w 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 by their 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 of shrouds for the same grmd: ng urnt, 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

SlX runs) one no-control nm was removed, 10 order 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:;a a 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 1,10 which 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 of time 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 appro Xlmate 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 t distnbunon

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 by weight

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 W g 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 by the 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 Safety and 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 of analynca; methods 4th rev ed , Eller PM, ed

Cmcmnan, OH U S Department of Health and Human Services, Public Health Service,

Centers for DISease Control and Prevention, National Institute for Occupational Safety

and Health, DHHS (NIOSH) Publication No 94-113

6 ACGrn [2001] Industrial venulanon, a manual of recommended 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

7
Table 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)

A Data not collected

+ Two control-off tnal were earned out ill block 1, one WIth the Metabo and one WIth the

Milwaukee gnnder

C A 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

{

8
Figure 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+---~.

. = ~ ~ i ~ ~ 2 ~ : 5 ~.!~ 1 ~ •

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