Transcript Nature of Air Contaminants

Occupational Air Sampling
Strategies – who, when, how….
Lecture Notes
IH&S 725 Dr. Myers, C.I.H.
Components of a Sampling
Strategy
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Characterization and information gathering
Risk assessment and sampling priorities
Air sampling strategy and analysis
Data interpretation
Recommendation and reporting
Re-evaluation
IH&S 725 Dr. Myers C.I.H.
Air sampling strategy



Which employee or employees should be
sampled?
How many samples should be taken on each
workday sampled to define the employee’s
exposure?
How long should the sampling interval be for
a measurement sample?
IH&S 725 Dr. Myers C.I.H.
Air sampling strategy


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What periods during the workday should the
employee’s exposure be sampled?
How many workdays during the year should
be sampled and when?
Time to result – acute vs. chronic and direct
reading real time vs. sampling media and
two-week lab time.
IH&S 725 Dr. Myers C.I.H.
Which employee or employees should
be sampled?


OSHA regulation requires the sampling of the
“employee believed to have the greatest
exposure” or the “maximum risk employee” principle extended to include groups of
employees
Use the exposure risk/health risk priority
matrix
IH&S 725 Dr. Myers C.I.H.
Which employee or employees should
be sampled?


If the maximum risk employee or group can’t
be identified then do random sampling of the
group of workers.
Objective is to select a subgroup of adequate
size so that there is a high probability that the
random sample will contain at least one
worker with high exposure if one exists.

Want to be careful about use of group statistics
IH&S 725 Dr. Myers C.I.H.
NIOSH’s Occupational Exposure
Sampling Strategies Manual

Gives one sample size
model
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
Set up to ensure with
90% confidence that at
least one person from the
highest 10% exposure
group is contained in the
sample
Conversely 10% chance
of missing someone in the
highest 10% exposure
group
IH&S 725 Dr. Myers C.I.H.
Sampling
periods


Various types of
sampling periods
possible
As you increase
the # of sample
periods in a shift
the analysis
becomes more
sophisticated
IH&S 725 Dr. Myers C.I.H.
How many workdays during the year
should be sampled and when?

OSHA regulation can differ

Substance specific standards specify sample
interval dependent on the exposure level relative
to the PEL
IH&S 725 Dr. Myers C.I.H.
1910.1025 Lead


1910.1025(c)(1) The employer shall assure that no
employee is exposed to lead at concentrations greater
than fifty micrograms per cubic meter of air (50
ug/m(3)) averaged over an 8-hour period.
1910.1025(c)(2) If an employee is exposed to lead for
more than 8 hours in any work day, the permissible
exposure limit, as a time weighted average (TWA) for
that day, shall be reduced according to the following
formula:

Maximum permissible limit (in ug/m(3))=400 divided by hours
worked in the day.
IH&S 725 Dr. Myers C.I.H.
1910.1025 Lead
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1910.1025 (d) Exposure monitoring 1910.1025(d)(1)(ii) With the exception of monitoring
under paragraph (d)(3), the employer shall collect full
shift (for at least 7 continuous hours) personal
samples including at least one sample for each shift
for each job classification in each work area.
1910.1025(d)(1)(iii) Full shift personal samples shall
be representative of the monitored employee's
regular, daily exposure to lead.
IH&S 725 Dr. Myers C.I.H.
1910.1025 Lead
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
1910.1025(d)(2) Initial determination. Each employer
who has a workplace or work operation covered by this
standard shall determine if any employee may be
exposed to lead at or above the action level.
1910.1025(d)(4)(i) Where a determination conducted
under paragraphs (d)(2) and (3) of this section shows
the possibility of any employee exposure at or above
the action level, the employer shall conduct monitoring
which is representative of the exposure for each
employee in the workplace who is exposed to lead.
IH&S 725 Dr. Myers C.I.H.
1910.1025 Lead
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1910.1025(d)(6) Frequency.
1910.1025(d)(6)(i) If the initial monitoring reveals employee
exposure to be below the action level the measurements need
not be repeated except as otherwise provided in paragraph (d)(7)
of this section.
1910.1025(d)(6)(ii) If the initial determination or subsequent
monitoring reveals employee exposure to be at or above the
action level but below the permissible exposure limit the employer
shall repeat monitoring in accordance with this paragraph at least
every 6 months. The employer shall continue monitoring at the
required frequency until at least two consecutive measurements,
taken at least 7 days apart, are below the action level at which
time the employer may discontinue monitoring for that employee
except as otherwise provided in paragraph (d)(7) of this section.
IH&S 725 Dr. Myers C.I.H.
1910.1025 Lead
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1910.1025(d)(6)(iii) If the initial monitoring reveals that employee
exposure is above the permissible exposure limit the employer
shall repeat monitoring quarterly. The employer shall continue
monitoring at the required frequency until at least two consecutive
measurements, taken at least 7 days apart, are below the PEL but
at or above the action level at which time the employer shall repeat
monitoring for that employee at the frequency specified in
paragraph (d)(6)(ii), except as otherwise provided in paragraph
(d)(7) of this section.
1910.1025(d)(7) Additional monitoring. Whenever there has been a
production, process, control or personnel change which may result
in new or additional exposure to lead, or whenever the employer
has any other reason to suspect a change which may result in new
or additional exposures to lead, additional monitoring in
accordance with this paragraph shall be conducted.
IH&S 725 Dr. Myers C.I.H.
Limited basic statistics review

What is a population?

What is a sample?
IH&S 725 Dr. Myers C.I.H.
Population and sample means
P o p ulat io n
Sam p le
n
N

x
i 1
N
i
x 
x
i 1
i
n
IH&S 725 Dr. Myers C.I.H.
Population variance and standard
deviation
P opulation
N
Varience   
2
 x   
i 1
2
i
N
Standard deviation   
2
IH&S 725 Dr. Myers C.I.H.
Sample variance and standard
deviation
Sample
n
Varience s 
2
 x  x 
i 1
2
i
n 1
Standarddeviation s  s
2
IH&S 725 Dr. Myers C.I.H.
Point estimate

A point estimate of some population
parameter is a single numerical value of a
statistic
IH&S 725 Dr. Myers C.I.H.
2-sided confidence interval on the
mean
CL1-


 s 
 x  t1 , n1  

2
 n
Two sided CI
P(L ≤ µ ≤ U) = 1-α where 0 ≤ α ≤ 1
IH&S 725 Dr. Myers C.I.H.
Confidence Interval on sample mean:
One - sided confidenceinterval
CL1-

 s 
 x  t1 , n1  

 n
α is assigned to either the upper or lower tail of the tdistribution
IH&S 725 Dr. Myers C.I.H.
Where does t come from

Definition

Let X1 X2 ….Xn be a random sample for a normal
distribution with unknown mean and unknown
variance σ2. The quantity
X 
T 
S
n
has a t-distribution with n-1 degrees of freedom.
IH&S 725 Dr. Myers C.I.H.
t-distribution and α



Probability density
function i.e.. area under
the curve = 1.
α represents the area
assigned to the tail(s) of
the distribution –
probability of T > t1-α
&/or -tα
Note that t1-α = -tα
IH&S 725 Dr. Myers C.I.H.
Example #1

Using the t-table determine the t-value with 14 df that
leaves an area of 0.025 to the left (and therefore an
area of 0.975 to the right).
t1  t  t0.975  t0.025
t0.975  2.145
IH&S 725 Dr. Myers C.I.H.
Example #2
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
What is the P(-t0.025 < T < t0.025)?
Since t0.025 leaves an area of 0.025 to the
right and -t0.025 leaves and area of 0.025 to
the left the total area (probability) between t0.025 and t0.025 is?
IH&S 725 Dr. Myers C.I.H.
Influence of degrees of freedom on the
t-distribution

The area to
the left or right
of tα and t1-α
decreases with
increasing df
IH&S 725 Dr. Myers C.I.H.
Standard normal distribution, SND

Definition

If is the mean of a random sample of size n taken
from a population with mean µ and variance σ2,
then the limiting form of the distribution of
Z
X 

n
as n
∞, is the standard normal distribution
snd(z;0,1).
IH&S 725 Dr. Myers C.I.H.
Using the standard normal
distribution

Using the z-table, find the area under the
curve that lies:

to the right of z = 1.84
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between z = -1.97 and z = 0.86
IH&S 725 Dr. Myers C.I.H.
Example #3

Given a normal distribution with µ = .05mg/m3
and σ = 0.01, find the probability that a sample
point estimate of µ would fall between 0.04 and
0.06mg/m3.
IH&S 725 Dr. Myers C.I.H.
Example #3 solution

First calculate the z-value corresponding to
x1 = .04mg/m3 and x2 = .06mg/m3
IH&S 725 Dr. Myers C.I.H.
Example #3 solution con’t

Second determine the probability

P(0.04mg/m3 < x < 0.06mg/m3) = P(-1< z < 1)
P( 1  z  1)  P( z  1)  P( z  1)
P  .8413 .1587
P  .6826or 68.3%
IH&S 725 Dr. Myers C.I.H.