6 - 10 Physics SBAs for the Primary FRCA
Question 6
Your ODP has just informed you that the team need to switch theatres for the next case because the current theatre is too humid.
Which of the following is true regarding humidity?
- Room air contains 17 g/litre water vapour
- The alveolus contains 44 g/litre water vapour
- The dew point is the temperature at which the partial pressure of water vapour and the saturated vapour pressure are equal
- Ultrasonic nebulisers operating at 2 MHz produce 15 microgram droplets
- Relative humidity is expressed in mg/litre and absolute humidity is expressed as a percentage
Answer
- The dew point is the temperature at which the partial pressure of water vapour and the saturated vapour pressure are equal
Room air contains 17 mg/litre or 17 g/m³ water vapour, and the alveolus has 44 mg/litre (or g/m³). The inspired air therefore undergoes humidification as it passes from room to respiratory tree.
Warm air can hold more water vapour than cold air.
- The absolute humidity is the amount of water vapour held by a volume of air, and is expressed in mg/litre or g/m³
- The relative humidity is the amount of water held by the air, as a percentage of the maximum it could theoretically hold
As you cool air down, its ability to hold water vapour decreases. Therefore assuming no water is being removed then by definition the relative humidity will steadily increase until it reaches 100%.
The temperature at which this happens is called the dew point, and below this, water will start to condense out as a liquid, hence the name.
Ultrasonic nebulisers operating at 2 MHz produce droplets less than 5 microns in size, which can deposit in the alveoli and lead to fluid overload, especially in children.