Download Measuring Air Changes per Hour with Carbon Dioxide

Measuring Air Changes per Hour
with Carbon Dioxide
Dr Rod Escombe MRCP DTM&H PhD
Honorary Research Fellow, Wellcome Centre for Clinical Tropical Medicine &
Department of Infectious Diseases & Immunity, Imperial College London, UK.
Senior Investigator, Asociación Benéfica PRISMA, Lima, Perú.
GP Registrar, St Mary’s Hospital, London, UK.
HOSPITAL
NACIONAL
“DOS DE MAYO”
Measuring room air exchange
• Measure air supply & extract
• Negative pressure: extract >
supply by ~10%
Calculating ACH
•
•
•
•
Q = room ventilation (cfm)
V = velocity (feet/min)
A = duct area (square feet)
ACH = Q/room volume
1
Measuring room air exchange in low
resource settings
Q=V x A
ACH = Q/volume
Measuring room air exchange using a
tracer gas
Principles:
•
•
•
•
Use gas that is not usually present in the room air: SF6
Deliver gas and mix with room air
Measure concentration decay to calculate air exchange
US$ 30,000
Also a continuous release methodology
Concentration decay
8000
7000
Standard tracer gas technique:
6000
5000
•
•
•
American Standards and Test Material Committee. 1988. Standard
test method for determining air leakage rate by tracer dilution. In
Annual Book ASTM Standards & Test Materials. Wash DC. 568575
Standard test methods for determining air change in a single zone
by means of a tracer gas dilution. Standard E741-00. Philadelphia,
Pennsylvania: American Society for Testing Materials.
Decker J. Evaluation of isolation rooms in health care settings
using tracer gas analysis. Appl Occup Envir Hyg 1995;10:887-91.
4000
3000
2000
1000
0
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
Ti m e
Other methods: theatrical fog
•
Gershey EL, Reiman J, Wood W, Party E. Evaluation of a room for
tuberculosis patient isolation using theatrical fog. Infect Control
Hosp Epidemiol. 1998 Oct;19(10):760-6.
2
Measuring natural ventilation
Measuring room air exchange using
carbon dioxide as a tracer gas
• Infra-red gas analyser to measure CO2 concentrations
• Advatanges:
Cheap
CO2 easily available (dry ice, fizzy drinks)
Can use occupied rooms
• Disadvantages:
Not perfect
CO2 present in air
CO2 produced by occupants
•
Menzies R, Schwartzman K, Loo V, Pasztor J. Measuring ventilation of patient care areas in
hospitals. Description of a new protocol. Am J Respir Crit Care Med 1995;152:1992-9.
3
CO2 release
Windows opened
6000
CO2 concentration (ppm)
5000
4000
Slow CO2 concentration decay
with windows closed
3000
Rapid decay with
windows open
0.5 ACH
2000
12 ACH
1000
0
0
5
10
15
20
25
30
35
Time (minutes)
Closing apertures
4
CO2 release
Small rooms, little ventilation:
Gas cylinder
Large rooms, lots of ventilation:
Fire extinguisher
CO2 release
Very big rooms – lots of fire extinguishers
5
Mixing: Aiming for
complete mixing
Mixing
no electricity
6
CO2 release
Windows opened
6000
CO2 concentration (ppm)
5000
4000
Slow CO2 concentration decay
with windows closed
3000
Rapid decay with
windows open
0.5 ACH
2000
12 ACH
1000
0
0
5
10
15
20
25
30
35
Time (minutes)
Calculation of ACH
CO2 release
Calculation of ACH
Windows opened
6000
ACH = Absolute ventilation / Room Volume
=Q/V
CO2 concentration (ppm)
5000
4000
3000
Slow CO2 concentration
decay with windows closed
Rapid decay
with windows open
0.5 air-changes/hour
12 air-changes/hour
2000
1000
0
0
5
10
15
20
25
30
35
Time (minutes)
Plot natural log of CO2 concentration
against time
ACH = slope of line of best fit
Ln ppm
Units: concentration (eg ppm)
vs. time in hours
Natural log of all data
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
0.00
0.20
0.40
0.60
0.80
hours from start of experiment
7
CO2 release
Windows opened
6000
Calculation of ACH
CO2 concentration (ppm)
5000
4000
3000
Slow CO2 concentration
decay with windows closed
Rapid decay
with windows open
0.5 air-changes/hour
12 air-changes/hour
2000
Natural logarithm CO2 concentration (ppm)
1000
0
Windows and doors CLOSED
0.5 ACH
0
5
10
15
20
25
30
35
Time (minutes)
8.7
y = -0.50x + 8.70
2
R = 0.89
8.6
8.5
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
Time (hours)
CO2 release
Windows opened
6000
Calculation of ACH
CO2 concentration (ppm)
5000
4000
3000
Slow CO2 concentration
decay with windows closed
Rapid decay
with windows open
0.5 air-changes/hour
12 air-changes/hour
2000
1000
0
0
5
10
15
20
25
30
35
Natural logarithm CO2 concentration (ppm)
Time (minutes)
9
Windows and doors OPEN
12 ACH
8
7
6
5
4
y = -12.25x + 12.10
R2 = 0.88
3
2
1
0
0.30
0.35
0.40
0.45
0.50
Time (hours)
8
Where to stop?
• Depends on room occupants
• Within 200 ppm of baseline
CO2 release
Windows opened
6000
CO2 concentration (ppm)
5000
4000
3000
Slow CO2 concentration
decay with windows closed
Rapid decay
with windows open
0.5 air-changes/hour
12 air-changes/hour
2000
1000
0
0
5
10
15
20
25
30
35
Time (minutes)
ACH vs. Absolute ventilation
Concentration (cfu/m3)
100
ACH = Q / V
Q = ACH x V
80
60
4m
V= 4x25 = 100 m3
12 ACH = 1,200 m3/h
2m
V = 2x25 = 50 m3
12 ACH = 600 m3/h
40
20
1 ACH
0
0
1
8
2
4
2
3
4
Area 25 m3
Time (hours)
In models of airborne infection, it is absolute ventilation that is a
major determinant of disease transmission
Wells-Riley equation: C=S(1-e –Iqpt/Q)
9
Effect of room volume if ACH is constant
Wells-Riley equation: C=S(1-e –Iqpt/Q)
ACH=12
Q=1,200
C=1x(1-e-1x13x0.6x8)/1,200
=0.05
Risk = 5%
ACH=12
Q=600
C=1x(1-e-1x13x0.6x8)/1,200
=0.10
Risk = 10%
C= new cases
S=susceptibles
I =number of infectors
q=infectious quanta produced per hour
p=pulmonary minute ventilation
t=duration of exposure
Q=absolute ventilation
Mechanical ventilation can’t deliver high ACH
in large rooms due to cost, drafts, fan noise etc
More complex: room crowding / bed spacing /
near-far effect
Measuring room air exchange using
carbon dioxide as a tracer gas
Conclusions
• Simple
• Easy to do
• Equipment relatively easily available
• Cheap
• A bit rough, but with inherent
variability of natural ventilation
(wind speed etc) and such high
air exchange rates, high precision
is less important
• It’s fun!
10
Thank you
11