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 4000 Annual Book ASTM Standards & Test Materials. Wash DC. 568- 3000 575 • Standard test methods for determining air change in a single zone 2000 by means of a tracer gas dilution. Standard E741-00. Philadelphia, 10 0 0 Pennsylvania: American Society for Testing Materials. 0 • Decker J. Evaluation of isolation rooms in health care settings 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 using tracer gas analysis. Appl Occup Envir Hyg 1995;10:887-91. Time 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
5000
4000
Slow CO concentration decay Rapid decay with 3000 2 with windows closed windows open 0.5 ACH 12 ACH 2000 concentration (ppm) concentration 2
CO 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
5000
4000
Slow CO concentration decay Rapid decay with 3000 2 with windows closed windows open 0.5 ACH 12 ACH 2000 concentration (ppm) concentration 2
CO 1000
0 0 5 10 15 20 25 30 35 Time (minutes) Calculation of ACH
CO2 release Windows opened
Calculation of ACH 6000 5000
4000
Slow CO2 concentration Rapid decay decay with windows closed with windows open 3000 0.5 air-changes/hour 12 air-changes/hour concentration (ppm) concentration
2 2000 ACH = Absolute ventilation / Room Volume CO 1000
= Q / V 0 0 5 10 15 20 25 30 35 Time (minutes)
Plot natural log of CO concentration 2 Natural log of all data against time 10.0 9.0 8.0 7.0 Units: concentration (eg ppm) 6.0 5.0
vs. time in hours Ln ppm 4.0 3.0 2.0 1.0 ACH = slope of line of best fit 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 5000 4000
Slow CO2 concentration Rapid decay decay with windows closed with windows open 3000 0.5 air-changes/hour 12 air-changes/hour concentration (ppm)
2 2000 CO
1000
0 Windows and doors CLOSED 0 5 10 15 20 25 30 35 Time (minutes) 0.5 ACH
8.7
y = -0.50x + 8.70 concentration (ppm) concentration 2 2 R = 0.89
8.6 Natural logarithm CO
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 5000 4000
Slow CO2 concentration Rapid decay decay with windows closed with windows open 3000 0.5 air-changes/hour 12 air-changes/hour concentration (ppm)
2 2000 CO
1000
0 0 5 10 15 20 25 30 35 Time (minutes)
9 8 Windows and doors OPEN 7 12 ACH 6 5 concentration (ppm) 2 4 y = -12.25x + 12.10 2 3 R = 0.88 2 1 0 Natural logarithmCO 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
5000
4000
Slow CO2 concentration Rapid decay decay with windows closed with windows open 3000 0.5 air-changes/hour 12 air-changes/hour concentration (ppm)
2 2000 CO
1000
0 0 5 10 15 20 25 30 35 Time (minutes)
ACH vs. Absolute ventilation
100
) 80 3 ACH = Q / V V= 4x25 = 100 m3 4m 3 60 Q = ACH x V 12 ACH = 1,200 m /h
40
20 1 ACH V = 2x25 = 50 m3 2m Concentration (cfu/m Concentration 12 ACH = 600 m3/h 0 2 4 012348 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
–Iqpt/Q C= new cases Wells-Riley equation: C=S(1-e ) S=susceptibles I =number of infectors q=infectious quanta produced per hour p=pulmonary minute ventilation Q=1,200 t=duration of exposure C=1x(1-e-1x13x0.6x8)/1,200 Q=absolute ventilation ACH=12 =0.05 Risk = 5%
Q=600 C=1x(1-e-1x13x0.6x8)/1,200 ACH=12 =0.10 Risk = 10%
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
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