Unit 6.1. Ventilation concepts;

natural ventilation

TB Infection Control Training for Managers

at National and Subnational Level

ObjectivesBy the end of this unit, participants will be able

to:• Describe the concept of ventilation• State the recommended air changes per

hour (ACH) for airborne precaution rooms• Calculate ACH• Utilize directional air flow to reduce the risk

of TB transmission• Describe how to maximize natural

ventilation2

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Outline

A. Ventilation concepts– Air changes per hour– Directional airflow– Types of ventilation systems

B. Natural ventilation– Wind– Stack

C. Exercise

A. What is ventilation?

• Movement of air

• “Pushing” and/ or “pulling” of particles and vapours

• Preferably in a controlled manner

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WHO recommends that health facilities implement

ventilation systems

• the better ventilated the area, the lower risk of transmission of TB and other airborne infections

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Air changes per hour (ACH)

• Calculating ACH is the most simple way to assess ventilation

• ACH = Volume of air moved in one hour

• One ACH means that the volume of air in the room is replaced in one hour

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Air changes per hour (ACH)

• WHO recommends at least 12 ACH to prevent airborne infection

• The higher the ACH, the better the dilution and the lower the risk of airborne infection

• But too much airflow can be uncomfortable (too much draft)

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ACH and time required for removal of 99% of droplet nuclei

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ACH, time required for removal of 99% and 99.9% of droplet nuclei

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Dilution ventilation

What do you need to measure ACH? 1.A tape

measure

2.Vaneometer

3.Smoke tube

4.Calculator

5.Note pad

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Measure dimensions of the opening to calculate area

Area of window opening = length x widthExample 1: Area = 0.5 m x 0.5 m = 0.25 m2

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Use the vaneometer to measure velocity, direction

Speed = metres per second = m/s13

Measurement of air velocity Measurement of air velocity through open window in through open window in KwaKwa--

Zulu Natal, South AfricaZulu Natal, South Africa

Never put fingers on the open space of the vaneometer

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Smoke tube to monitor air flow Smoke tube to monitor air flow through a door into a sputum through a door into a sputum

collection booth in Riga, Latviacollection booth in Riga, Latvia

Is air flowing the right direction?15

Calculate air flow rate

Example 1:Air velocity through window measured by

vaneometer = 1 m/sFlow rate = Open window area x air velocity = 0.25 m2 x 1 m/second

= 0.25 m3/s x 3,600 seconds per hour= 900 m3 / hour

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Room volume = width x depth x height

Example 1:

3 m wide x 5 m deep x 3 m high = 45 m3

Calculate room volume

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Example 1: ACH calculation

• Window area = length x width = 0.25 m2

• Air velocity through window= 1 m/s• Air flow rate = window area x air velocity = 900 m3/h• Room volume = width x depth x height = 45 m3

• ACH = Air flow rate divided by room volume

= 900 m3/hour = 20 ACH 45 m3

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Bed

Bed

WindowW

indo

wWindow closed

Win

dow

clo

sed

Each open window = 1m x 1m = 1m2

Door

Example 2: ACH calculation

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Area of open windows

= 2 m2

Bed

Bed

Window

Win

dow

Window closed

Win

dow

clo

sed

1 m2 1

m2

Door

20

0.20 m/s

0.10 m/s Average air velocity =

0.20 +0.10 m/s

2

=0.15 m/sec

Bed

Bed

Window

Win

dow

Window closed

Door

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Average air velocity 0.15 m/sec

Average Flow Rate =

Average air velocity 0.15 m/sec

X Area of windows 2 m2

X 3,600 sec/h

= 1,080 m3 / h

Bed

Bed

Window

Win

dow

Window closed

Door

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Room volume:

4.5 m x 4 m

x 3.5 m= 63 m3

ACH = flow rate room volume

= 1,080 m3 / h 63 m3

= 17 ACH

Bed

Bed

Window

Win

dow

Door

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ACH examples Room volume Average air

flowrate (Air volume x h)

ACH

4m x 4m x 2.5 m m3 x h Flow/ volume

40 m3 40 1

40 m3 200 5

40 m3 300 7.5

40 m3 600 15

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Window openings

Ventilation is more effective if:

1. Air flows from “clean” to “contaminated” (directional airflow)

2. There is good air-mixing (no stagnation or short circuiting)

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Directional airflow

Locate the health care worker (or other patients) near the clean air source

Locate the person who may be infectious near a place where the air is exhausted away

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Correct working location

Health care worker (HCW) is near the clean air source

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Incorrect working location

Resolve by switching places so the health care worker is near the clean air source

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Good compromise

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Types of ventilation

1. Natural

• If feasible, maximize the use of natural ventilation before considering other ventilation systems.

2. Mechanical

3. Mixed mode

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B. Natural ventilation

Created by the use of external airflows generated by natural forces such as:

• Wind

• Differences in temperature (stack)

Naturally ventilated rooms can achieve very high ventilation rates (ACH) under ideal conditions

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Natural ventilation

Open Window

Open Window

Door C D E F

BDirection of air flow

Beds

A

Beds

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Maximize natural ventilation

• Openings on opposite walls (cross ventilation)• Openings are unrestricted (stay open)• 10% of floor space should be openable

window area on each wall• Upper levels of the building (higher from the

ground floor)• Building and openings are oriented to use the

prevailing wind, without obstruction by other nearby buildings

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Stack ventilation

Turbine driven ventilation (whirly bird)

Natural ventilationAdvantages• Can often be implemented immediately• Often low cost• Can achieve high ACH

Disadvantages• Uncontrolled• Unpredictable• Safety, comfort• Insects, noise, dust• Not suitable in cold weather

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Summary

• WHO recommends at least 12 room air changes per hour to prevent airborne infection

• Locate the health care worker (or other patients) near the clean air source

• Locate the patient who may be infectious near a place where the air is exhausted away

• In existing health care facilities with natural ventilation, the use of natural ventilation should be maximized before considering other ventilation systems.

Exercise

• How would you improve the ventilation of the following health facility, using natural ventilation concepts?

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Poorly ventilated waiting area in an out-patient clinic

Side A

Side B

Office

Office

Office

Pharmacy

Exa

min

atio

n r

oom

s

X X X X

Doors in/ out

Doors in

Plane view Front viewChairs, waiting area

Windows

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Waiting area maximizing natural ventilation

Side A

Side B

Office

Office

Office

Pharmacy

Exa

min

atio

n r

oom

s

Doors in/ out

Plane view Front view

Wai

tin

g a

rea

3 Windows tranformed into doors

Side B

Waiting area

45

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