Assessing particle deposition rates is complicated due to the limitations of accurately counting particles above 1 micrometre in diameter. However, specialised studies can reveal the extent of risk that these particles pose. Regular cleaning and disinfection are important for addressing floor level contamination.
For these reasons, a cleanroom changing room requires:

• A limit on operator numbers
• Relatively high air rates (clean air supply over a person, as measured in cubic metres per minute)
• Good turbulent flow
• Use of appropriate clothing
• A rest period between one group of personnel leaving and the next group entering

Operator numbers

The number of operators permitted in a cleanroom changing room will depend on several factors. Generally, the space that an operator occupies will need to be larger than the minimum spaces allocated for conventional changing rooms. For a standard space (such as a swimming pool changing cubicle) many countries stipulate a space of 1 m2 per person. For a cleanroom changing room, it is often recommended that 4m2 is allocated (accounting for the indoor concentration gradient created from an individual’s ‘personal activity cloud’). However, this is dependent on the other factors.

It is also known that each additional person into any cleanroom space increases the particle concentration as well as the opportunity for particle mass exchange (the transfer of contamination from one person to another). The rate of particle concentration increase will depend on the type of clothing worn (and for how long) and the rate of operator movement. Deposition also comes in since most larger particles fall to the floor due to gravity, although this can be enhanced through air turbulence. Estimating particle release is not straight forward and localised body emissions need to be considered in combination with simplified bodily movements and accounting for all areas of the body (including the mouth and nose ⁴). 

Therefore, we have:

• Number of persons performing the gowning activity 
• The vigour of the activity
• The type of activity
•  The type of flooring

Without cleanroom clothing, a typical human releases 8 million particles/h in the diameter range 1-10 μm [4] in a sitting position. In total, humans shed around one billion skin cells each day ⁵.

As well as generating particles, operator activity also resuspends particles through contact with the floor. The primary mechanism of particle resuspension involves rolling detachment adhesive forces can be greatly reduced by microscopic surface roughness. While footwear contact can resuspend a level of particles, other activities that can help to scoop up particles and increase the concentration, such as the cleanroom gown contacting the floor, must be avoided (resuspension is a significant contaminant re-distributor) ⁶. This is necessary to minimise the cross-contamination of personnel, especially those who have donned the cleanroom gown.

Use of appropriate clothing

Wearing clean clothing, such as a cleanroom smock or jumpsuit, on entry to the changing room significantly reduces the particle load compared with personnel accessing a cleanroom wearing outdoor clothing ⁷. Under the EU GMP framework, this is required for entry into Grade C cleanrooms, although based on particle reduction it is recommended for prior entry into any controlled environment. 

Once the cleanroom garment is worn, contamination release is substantially lowered. Cleanroom synthetic garment systems show a better performance, achieving 10 to 30 times fewer particle emission rates in wider movements and almost zero microbiological release compared with standard clean clothing. Not all cleanroom garments are of the same quality - they differ by material, fabric warping and sewing, shape and donning ⁸. Emission rates tend to be lower for disposable garments compared with reusable ones, although operator comfort, as well as cost, needs to be balanced.

Environmental control

Keeping temperature relatively cool (such as 15 to 21oC) reduces operator perspiration. This, along with appropriate humidity, can contribute towards control. High humidity can also lead to easier particle resuspension ⁹.

Excluded activities

As well as excessive movement, activities like the cleaning of changing rooms should not take place whilst personnel are in the room for gowning or degowning. A simple process like dry-mopping the floor will increase the total particle count by five-fold ¹⁰. Normal cleanroom airflow generally does not resuspend particles - it is operator activities that are the cause ¹¹.

Summary

This article has looked at some of the risk factors that affect cleanroom gowning and the use of changing rooms. The biggest risk factors are the types of clothing worn, the number of operators, the speed of activity and the gown potentially touching the floor.