Spirometry Quality Assurance: 

Common Errors and Their 

Impact on Test Results 
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 2 
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 3

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DHHS (NIOSH) Publication No. 2012-116

January 2012
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 4 

Acknowledgements

The principal contributors to this document were Lu-Ann F. Beeckman-Wagner, Ph.D. and

Diana Freeland, LPN, CPFT. Special thanks to Kimberly Clough Thomas, BFA for her skills in

document design and Mansi Shah Das, MPH, MBA for her health communication expertise.

Internal NIOSH reviewers who provided critical feedback to the preparation of the document: 

Robert M. Castellan, M.D., M.P.H. Kristin Cummings, M.D., M.P.H. 

External Expert Peer Review Panel

NIOSH expresses appreciation to the following independent, external reviewers for providing insights 

and comments that contributed to the development of this document and enhanced the final version:

Brian Boehlecke, M.D., M.P.H. John L. Hankinson, Ph.D.

University of North Carolina School of Medicine Hankinson Consulting, Inc.

Chapel Hill, NC Athens, GA

Robert Cohen, M.D., F.C.C.P. Mary C. Townsend, Dr. P.H.

Cook County Health and Hospital’s System M.C. Townsend Associates, LLC

Chicago, IL Pittsburgh, PA

Christopher Martin, M.D., M.Sc.

West Virginia University School of Medicine 

Morgantown, WV 
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 5

Table of Contents

Spirometry Quality Assurance: Common Errors and Their Impact on Test Results

Valid Normal Test

Error #1: Sub-maximal Inhalation 

Error #2: Excessive Extrapolated Volume

Error #3: Sub-maximal Blast

Error #4: Cough in First Second

Error #5: Early Termination 

Error #6: Variable Effort 

Error#7: Cessation of Airflow – Glottis Closure or Breath Holding 

Error #8: Partially Obstructed Mouthpiece 

Error #9: Leak

Error #10: Extra Breath(s)

Error #11: Positive Zero-Flow Error 

Error #12: Negative Zero-Flow Error 

6

8

10

12

14

16

18

20

22

24

26

28

30

32
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 6 

Spirometry Quality Assurance: 

Common Errors and Their Impact on Test Results 

Spirometry is the most common pulmonary function test (PFT) and it plays a central role 

in occupational respiratory disease surveillance programs. Accurate spirometry testing, 

interpretation, and follow-up are critical to effective screening and surveillance of workers 

exposed to respiratory hazards and to the management of patients in the clinical setting. 

Spirometry quality assurance includes examination of test values and evaluation of both 

the volume-time and flow-volume curves for evidence of technical errors. Technically poor 

spirometry may have little value and may even provide misleading information. When 

erroneous curves are detected, additional maneuvers are often needed. During testing, 

technicians should attempt to record a valid test, which is composed of at least 3 acceptable 

maneuvers with consistent (“repeatable”) results for both FVC and FEV1. Achieving repeatability 

during testing means that the difference between the largest and second largest values for 

both FVC and for FEV1 are within 0 .15 l (150 ml). Additional maneuvers can be attempted, up 

to a maximum of 8, to meet these criteria for a valid test. Even tests that have 3 acceptable 

maneuvers of which at least 2 are repeatable may not represent an individual’s maximum effort. 

Ideally, there should be 3 maximal effort curves with the same size and shape.

This guide depicts common testing errors and shows how these errors can impact spirometry 

results. However, health professionals must bear in mind that, though the goal during testing 

is to obtain a valid test as described above, results from invalid tests may sometimes provide 
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 7

List of Abbreviations:

BTPS = body temperature and pressure saturated

FEV1 = forced expiratory volume in one second 

FEV1/FVC = ratio of forced expiratory volume in one second compared to the total 

volume of air exhaled, expressed as a percent

FVC = forced vital capacity 

PEF = peak expiratory flow

% Pred = percent of predicted value

LLN = lower limit of normal

Vext = extrapolated volume

information that can be used to infer respiratory health status. In 1994, the American Thoracic 

Society* stated that tests should not be interpreted if fewer than 2 acceptable curves were 

recorded, reflecting the fact that useful information may be present even in tests that are 

not optimal. In fact, some medical conditions prevent workers and other test subjects 

from successfully recording valid tests, in spite of the best efforts of the technician and the 

subject to produce such results. Clinical judgment should be used to determine whether 

curves are so unacceptable that they would lead to a misinterpretation of respiratory health. 

The first figure represents a valid normal spirometry test including 3 curves, each produced 

by a separate expiratory maneuver (trial). All subsequent figures with examples of testing 

errors show one acceptable trial (green – solid line) together with one or more trials with 

the error (red – dashed line) for comparison. If more than one red trial is presented, then 

the test results from the trial with the largest values are reported. Only test results that are 

impacted by the specific error will be shown in the tables for these examples.

In the following figures, FEV1 is indicated by a vertical mark 1 second after the start of the volumetime curves, and arrows are used to emphasize key features of the curves in each example.

*American Thoracic Society. Standardization of Spirometry: 1994 Update. Am J Respir Crit Care Med 1995:152;1107-1136.
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 8 

Valid Normal Test

Trial FVC (L) FEV1 (L) PEF (L/sec)

1 4.81 4.09 12.1

2 4.74 4.07 12.0

3 4.87 4.14 12.5

Repeatability 0.06 0.05

4.87-4.81 = 0.06 4.14-4.09 = 0.05
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 9

A valid test has at least 3 acceptable trials and both the FVC and FEV1 are repeatable [i.e., 

the two highest values from acceptable maneuvers are within 0.15 L (150 ml)]. 

The volume-time curve (left) shows the time course of exhalation, starting from the 

baseline (i.e., zero) volume, rising sharply, and then gradually slowing until a 1-second 

FVC plateau is drawn, indicating the end of exhalation. The volume-time curve allows 

practitioners to evaluate the completeness of the end of the test. The American Thoracic 

Society* recommends that subjects try to exhale for at least 6 seconds; however, young 

healthy adults may empty their lungs before 6 seconds. No trial should be deleted on the 

basis of exhalation time as the FEV1 may still be valid. A volume-time curve that does not 

plateau before 15 seconds despite maximal effort by the subject may indicate “obstructive 

impairment.” Also, it may be difficult to reach a plateau on the volume-time curve for some 

subjects who cannot sustain their expiration. This may happen in obese subjects or those 

with some underlying lung diseases. The flow-volume curve (right) emphasizes patient 

effort and shows detailed information at the beginning of the test. It is characterized by: 

1) an immediate vertical rise, 2) a sharp peak, and 3) a fairly smooth descent that returns 

to zero flow. 

In this guide, the first second of exhalation is designated by a black vertical line crossing 

the volume-time curve. Not all spirometry displays have a 1-second marker.

*American Thoracic Society/European Respiratory Society. Standardization of spirometry. Eur Respir J 2005:26:319-338.
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 10 

Error #1: Sub-maximal Inhalation

FVC (L) FVC % Pred FVC LLN (L) FEV1 (L) FEV1 % Pred FEV1 LLN (L)

Good Effort 4.90 96 4.17 4.17 103 3.29 

Error 4.18 82 4.17 3.60 88 3.29 

4.90-4.18 = 0.72
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 11

Problem: Sub-maximal inhalation causes underreporting of true spirometry results. An 

incomplete inhalation is a frequent and serious problem that occurs in spirometry testing.

Identification: Lack of full inspiration can be seen as a gap between the FVC plateaus 

on multiple volume-time curves (left) and also as a space between the ending points of 

multiple flow-volume curves (right). The shape of both types of curves will be similar, 

but the curves will vary in size. Flow-volume curves with sharp peaks indicate that the 

efforts have good initial blasts, but the difference in maximal airflow (PEF) may be 

caused by lower elastic recoil associated with less “stretching” of the lung with smaller 

volumes.

Test Result Implications: Falsely reduced FVC can be misinterpreted as indicating a 

“restrictive impairment.” Results from tests with varying inspirations may fail to achieve 

the repeatability needed for a “valid” test. Although in this example we do not have a 

repeatable test, a test interpretation of “normal” is still valid. An incomplete inhalation 

results in falsely low values which may be below the LLN. However, in this example, the 

FVC and FEV1 values are above the LLN and the results appear “normal.”

Solution: Coach the subject to FILL THEIR LUNGS, taking the deepest possible breath.

Spirometer Error Message: “FVC variable”, “FEV1 variable”, or “Take a deeper breath.”
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 12 

Error #2: Excessive Extrapolated Volume

FVC (L) FEV1 (L) FEV1 % Pred PEF (L/sec) Vext (L) Vext (%)

Good Effort 4.79 4.12 86 12.2 0.12 2.5 

Error 4.78 3.95 82 8.5 0.55 11.5 
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 13

Problem: A slight hesitation in blowing out before the initial blast affects most spirometry 

test results early in the maneuver. A maneuver showing this error must not be used 

to measure FEV1 but it may be used to validate the FVC if it is consistent with other 

acceptable, but less than perfect curves. 

Identification: Since hesitation occurs early in the test, it is most easily seen in the 

flow-volume curve (right), which has its peak flow displaced to the right. The spirometer 

makes a measurement called “extrapolated volume” (Vext) to determine whether a 

hesitation is excessive. The extrapolated volume is unacceptable if it exceeds 0.15 

liters or 5% of the FVC, whichever is larger.

Test Result Implications: Large extrapolated volume can falsely elevates the FEV1, 

though occasionally the FEV1 may be reduced. 

Solution: Coach the subject to blast FASTER or IMMEDIATELY.

Spirometer Error Messages: Most spirometers label this error with “Hesitation,” 

“Large extrapolated volume,” or “Start faster.”
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 14 

Error #3: Sub-maximal Blast

FVC (L) FEV1 (L) FEV1 % Pred PEF (L/sec) 

Good Effort 4.69 4.08 86 12.6 

Error 4.57 3.36 71 4.1
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 15

Problem: A poor initial blast affects spirometry measurements made early in the 

maneuver. 

Identification: This problem is most clearly seen on the flow-volume curve (right). 

The peak on the curve is reduced, indicating insufficient subject effort — the weaker 

the blast, the lower the peak (PEF). On the volume time curve (left), a weak blast 

prevents the curve from rising sharply from the volume baseline.

Test Result Implications: A curve with low peak flow will have falsely reduced 

FEV1 and FEV1/FVC ratio that may be misinterpreted as “obstructive impairment” 

if other good curves are not available. A repeatable test (both the FVC and FEV1) 

may occur with sub-maximal effort. In this example the FVC is repeatable but 

the FEV1 is not.

Solution: Coach the subject to blast the air out HARDER.

Spirometer Error Messages: Spirometers may not label these curves as errors, so 

health professionals must recognize these patterns. 
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 16 

Error #4: Cough in First Second

FEV1 (L) FEV1 % Pred 

Good Effort 4.12 88

Error 3.96 84
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 17

Problem: Coughing in the first second may affect the FEV1 measurement, while coughing 

after the first second will not affect the FVC measurement unless the subject stops blowing 

prematurely. A maneuver showing this error must not be used to measure FEV1 but it may 

be used to validate the FVC if it is consistent with other acceptable, but less than perfect 

curves. 

Identification: Cough is most clearly seen as a jagged interruption in the flow-volume curve 

(right). Although this example has one cough, often there are multiple coughs in a single 

maneuver. Cough is more difficult to see on the volume-time curve. 

Test Result Implications: A cough can cause either a falsely reduced or falsely elevated 

FEV1, depending on the strength of the cough. However, a cough is difficult to define and its 

appearance on the display is very instrument-dependent. Significant coughing may cause a 

falsely reduced FVC. In any case, because the FEV1/FVC ratio may be inaccurate, the maneuver 

is unacceptable.

Solution: Coughing is difficult to manage. Offering a drink of water before the maneuver 

may help.

Spirometer Error Message: Some spirometers label this error with “Cough.”
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 18 

Error #5: Early Termination

FVC (L) FVC % Pred FEV1 (L) FEV1 % Pred FEV1/FVC% 

Good Effort 4.54 83 3.91 87 86 

Error 3.81 67 3.76 84 98 
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 19

Problem: Subjects who do not completely exhale have falsely reported spirometry 

results.

Identification: Early termination is most clearly seen as a lack of plateau on the 

volume-time curve (left). Most healthy adults can exhale for >6 seconds, and more 

time is required to reach the plateau as subjects age or develop airways obstruction. 

However, many young healthy subjects exhale to their plateaus within 3 to 5 seconds 

and these are valid tests. If the maneuver is terminated early enough, this error can 

also be seen as a sharp drop to zero flow on the flow-volume curve (right).

Test Result Implications: The falsely reduced FVC may be misinterpreted as indicating a 

“restrictive impairment.” The falsely increased FEV1/FVC ratio may cause a true “obstructive 

impairment” to be missed. So, an interpretation of “no restrictive impairment” may be 

accurate if the FVC is above the LLN. However, since the FEV1/FVC ratio may not be valid, 

one cannot rule out an obstructive pattern.

Solution: Coach the subject to KEEP BLOWING until told to stop.

Spirometer Error Message: Spirometers may label this error as “Early termination” 

or “Keep blowing”. Other spirometers may not label these curves as errors, so health 

professionals must recognize these patterns.
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 20 

Error #6: Variable Effort

FEV1 (L) FEV1 % Pred FEV1/FVC% PEF (L/sec) 

Good Effort 3.73 86 88 10.8 

Error 3.49 80 82 4.8 
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 21

Problem: Exhaled airflow varies substantially if effort is variable, particularly during the 

first 2-3 seconds of the maneuver.

Identification: While coughs are usually seen as jagged interruptions in the flowvolume curve, variable effort is often seen as a dip in the flow-volume curve (right). 

The more variable the effort, the larger the dip on the flow-volume curve.

Test Result Implications: The falsely reduced FEV1 and FEV1/FVC ratio may be 

misinterpreted as indicating an “obstructive impairment.” However, the shape 

of the flow-volume curve and the reduced PEF indicate that the low values are 

caused by poor effort.

Solution: Coach the subject to blast one breath out HARD and FAST, and KEEP 

BLOWING out. 

Spirometer Error Message: Many spirometers do not label this error, so health 

professionals must recognize these patterns.
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 22 

Error #7: Cessation of Airflow – Glottis Closure or Breath Holding

FVC (L) FVC % Pred FEV1 (L) FEV1/FVC % 

Good Effort 4.90 91 4.16 85

Error 4.40 82 4.16 96 
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 23

Problem: Airflow during exhalation may suddenly cease before the lungs have been 

completely emptied.

Identification: Closing the vocal cords (glottis closure) and breath holding both cause the 

volume-time curve to show an abrupt horizontal line (left). The flow-volume curve drops 

sharply to zero flow (right). 

Test Result Implications: The FVC is falsely reduced and may be misinterpreted as 

indicating a “restrictive impairment.” In addition, the FEV1/FVC ratio may be falsely 

elevated, resulting in normal FEV1/FVC even when the subject has a mild obstructive 

impairment.

Solution: Glottis closure may be involuntary and should be documented. However, 

for breath holding, coach the subject to blow UNTIL TOLD TO STOP.

Spirometer Error Message: Some spirometers will label this error with “Blow out 

longer” or “Abrupt stop.”
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 24 

Error #8: Partially Obstructed Mouthpiece

FEV1 (L) FEV1 % Pred FEV1/FVC% PEF (L/sec) 

Good Effort 4.08 101 87 12.4 

Error 3.09 77 68 7.2 
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 25

Problem: Speed of exhaled air is reduced if the mouthpiece is partially blocked by the 

subject’s tongue, teeth, loose dentures, or strong biting that deforms the shape of the 

mouthpiece. 

Identification: Blockage of the mouthpiece can cause flow-volume and volume-time 

curves to be misshapen in a variety of ways. The flow-volume curve may have a 

reduced peak flow and portions of the curve are flattened (right). The volume-time 

curve may show some flattening after the initial rise of the curve (left). The specific 

shape of each curve depends upon when and to what degree airflow was reduced — 

the greater the degree of mouthpiece obstruction, the flatter the curve. 

Test Result Implications: The FVC may be reduced if there is sufficient pressure to 

cause air loss around the mouthpiece. The falsely reduced FEV1 and FEV1/FVC ratio 

may be misinterpreted as indicating an “obstructive impairment.” 

Solution: Coach the subject to place the mouthpiece between the teeth and on top 

of the tongue. Securing dentures may be preferable to removing them, as it maintains 

anatomical integrity and promotes proper lip seal. Lightly biting on (but not deforming) 

the mouthpiece usually helps unless the dentures are very poorly fitted.

Spirometer Error Message: Many spirometers will not label these curves as erroneous, 

so health professionals must recognize these patterns.
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 26 

Error #9: Leak

FVC (L) FVC % Pred FEV1 (L) FEV1/FVC% 

Good Effort 4.71 80 4.05 86 

Error 4.35 74 4.05 93
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 27

Problem: Leaks in volume-type spirometers equipped with auto-negator springs will cause 

volume and flow measurements to be falsely low. The leaks can occur in the spirometer, 

hose, or around the subject’s mouthpiece during a maneuver.

Identification: When leaks are large enough to be visualized, they cause the volume-time 

curve to descend after a plateau is reached (left). They also cause the flow-volume curve 

to “back-track” toward zero volume at the end of the maneuver (right). 

Test Result Implications: When leaks are visible in either graph, the effect on the FVC is 

likely to be profound. Since the FEV1 is not much affected, the FEV1/FVC ratio is often 

falsely elevated. These false results may prevent the detection of “obstructive impairment” 

or be misinterpreted as indicating a “restrictive impairment.” 

Solution: Leaks are more common in volume spirometers. The technician must follow the leak check 

protocol to determine if it is in the spirometer, the hose, or at the mouthpeice. A flow spirometer 

system can leak, but will not cause graphs like those displayed. A leak will cause the flow spirometer 

to fail its calibration check when the syringe is injected slowly. A technician using a flow spirometer 

needs to check for loose connections or for holes or wear in tubing. Technicians should also ensure 

that a tight lip seal is maintained by the subject since some dental devices may interfere with a 

proper lip seal. An astute technician can usually hear air escaping around the mouthpiece. 

Spirometer Error Message: Spirometers do not label this error; it must be detected during a 

calibration check.
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 28 

Error #10: Extra Breath(s)

FVC (L) FVC % Pred FEV1/FVC% 

Good Effort 4.43 97 88 

Error 5.55 122 69 
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 29

Problem: Subjects sometimes take extra breaths through their nose or around the 

sides of the mouthpiece at or near the end of the maneuver. 

Identification: Extra breaths are easily seen in both displays: the volume-time curve 

has “steps” added onto the plateau at the end of the exhalation (left) and the flowvolume curve has small curves added after the large curve (right). There may be one or 

more extra breaths with the artifact size dependent on the amount of air breathed in.

Test Result Implications: The FVC is falsely elevated, though the FEV1 is not 

affected, causing a falsely reduced FEV1/FVC ratio, which may be misinterpreted 

as indicating an “obstructive impairment.” 

Solution: Use nose clips and instruct the subject to keep a tight lip seal around the 

mouthpiece. Since these extra breaths falsely elevate the FVC, the curves must be 

deleted and not saved with the test results. 

Spirometer Error Message: Spirometers do not label this error, so health professionals 

must recognize these patterns.
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 30 

Error #11: Positive Zero-Flow Error

FVC (L) FVC % Pred FEV1 (L) FEV1 % Pred FEV1/FVC% 

Good Effort 4.41 85 3.87 95 88 

Error 5.68 109 3.93 96 69 
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 31

Problem: A positive zero-flow error (in a flow-type spirometer) causes over-recording of all 

expiratory measurements. This error can occur when the zero flow reference point is set 

incorrectly.

Identification: Positive zero-flow errors may cause the volume-time curve to rise at a 

constant rate and never plateau (left). The angle of the rise depends on the severity of 

the zero error — the greater the error, the steeper the slope. The flow-volume curve 

shows a long tail at the end of expiration that remains at constant positive flow (right). 

Test Result Implications: The FVC is more affected than the FEV1 causing a falsely reduced 

FEV1/FVC ratio which may be misinterpreted as indicating an “obstructive impairment.” 

Solution: Many flow-type spirometers are susceptible to zero-flow errors. To prevent 

these errors, the technician should block the sensor outlet to prevent air motion through 

the sensor while the zero flow reference is set. Sensors should be held upright and still 

during subject testing. Some spirometers determine zero flow before each maneuver, 

whereas other spirometers measure it only once before a subject’s set of maneuvers. Zero 

errors are likely to vary in the former case, but curves will be consistently incorrect in the 

latter case. All curves recorded with zero errors must be deleted.

Spirometer Error Message: Spirometers do not label these errors, so health 

professionals must recognize these patterns.
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 32 

Error #12: Negative Zero-Flow Error

FVC (L) FVC % Pred FEV1 (L) FEV1 % Pred FEV1 /FVC% 

Good Effort 4.41 85 3.87 95 88 

Error 3.81 73 3.63 89 95 
2012 NIOSH Spirometry Quality Assurance: Common Errors and Their Impact on Test Results 33

Problem: A negative zero-flow error (in a flow-type spirometer) can cause under-recording 

of all expiratory measurements. The degree to which under-recording occurs depends on 

the severity of the zero flow error. This error can occur when the zero flow reference point 

is set incorrectly.

Identification: A negative zero-flow error may cause the volume-time curve to end 

abruptly or it might drop gradually toward zero volume (left), similar to a large leak. 

Test Result Implications: The FVC is more affected than the FEV1, causing a falsely 

increased FEV1/FVC ratio. These results may be misinterpreted as indicating a 

“restrictive impairment” or might obscure a true “obstructive impairment.” 

Solution: Many flow-type spirometers are susceptible to zero-flow errors. To prevent 

these errors, the technician should block the sensor outlet to prevent air motion through 

the sensor while the zero flow reference is set. Sensors should be held upright and still 

during subject testing. Some spirometers determine zero flow before each maneuver, 

whereas other spirometers measure it only once before a subject’s set of maneuvers. Zero 

errors are likely to vary in the former case, but curves will be consistently incorrect in the 

latter case. All curves recorded with zero errors must be deleted.

Spirometer Error Message: Spirometers do not label these errors, so health 

professionals must recognize these patterns.
DHHS (NIOSH) Publication No. 2012-116

Department of Health and Human Services

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

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Cincinnati, OH 45226-1998

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