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Sock to sock:

Mitigating the risk from personal socks for the cleanroom change procedure

By Tim Sandle - PhD

Introduction

Microorganisms are found in different numbers and there is species diversity across different regions of the human body. One area where there is a high concentration of microorganisms is the human foot(1). It is due to the high levels and relative ease of transmission that the revision to EU GMP Annex 1 has included a requirement for cleanroom socks to be worn prior to entry into changing rooms that lead into cleanrooms. This is a response to those who elect to walk around normally in bare feet and from the transmission potential from everyday socks.

But how are cleanroom socks to be used and how can the contamination level be controlled? This paper looks at the risk factors and provides a straightforward solution.

The use of cleanroom socks

Given the importance of cleanroom socks to meet regulatory expectations and given the concern with the microbial load on feet and socks, a question that has arisen, however, is that if there is a high concentration of microorganisms on the foot, how should the cleanroom sock be worn? By removing the outdoor sock, there exists the potential to disperse microbial-carrying particles into the airstream (2) and then risk of cleanroom operators picking up contamination on their hands. A better solution would appear to be the use of cleanroom socks that are pulled over the personal sock, rather than needing to remove the personal sock.

It is not straightforward to quantify the proportion of particles released when socks are removed, and this is an under-researched area. Studies involving the removal of clothing for patients in hospitals suggest that such clothing serves as a reservoir of particles (3), including bioaerosols, which may have health significance (4).

In any circumstance, human particle and microbiological emission rate will depend on sock quality and the relative human activity vigour, with both factors influencing the total particle release in clean environments (5). Given that many facilities will expect personnel to move through the first stage of changing relatively quickly, the slow and deliberate movement of socks, followed by hand sanitisation, and then the careful donning of cleanroom socks using a technique designed not to touch the feet would prove difficult to enforce and to practice (as would the wearing of cleanroom gloves, which may be effective (6) but would increase costs). Furthermore, control measures will be more difficult to enforce in more densely occupied spaces given the interactions between each individual’s personal particle cloud (7). First stage changing rooms are not often designed the same way as final stage changing rooms, in terms of air dynamics, leading to unfavourable velocity fields that can increase personal exposure to airborne particles (8).

Microorganisms and human feet

There is a high abundance of microorganisms found on human feet. The primary reason for this relates to the ecology of this region of the body. The greatest number of sweat glands are found in foot than any other part of the body. When shoes and socks are worn, this prevents sweat from evaporating or being absorbed and the resultant environment favours the growth of both bacteria and fungi (9). In addition, wearing socks in shoes allows less air circulation than in other areas of the body (10). While this may alter over time (11), the region of the foot tends to continuously carry a relatively high concentration of microorganisms. The resultant odour inside a sock or shoe will increase across time due to accumulation of dead skin cells, dirt and oils (12).

Given the predominance of organisms on the foot, either a sizable number will be deposited inside the sock and be released when socks are removed , or they will be transferred onto the hands.

Microorganisms associated with socks

The types of microorganisms recovered from sock will vary according to differences between people and according to geographical areas (especially in terms of temperature and humidity differences, or the overall thermoregulatory effectiveness of individual socks) (13,14). A further factor will include the type of textile. Nylon, for example, promotes growth more than cotton or polyester. Nylon as a sock material is classed as a non-absorbent and this property causes the perspiration to remain in contact with the feet favouring bacterial growth. Even when superior materials are used, such as cotton, no two textiles are the same (15). Taking just one aspect, the moisture management properties of socks and the layered textiles used, these will vary according to the fibre composition and geometrical properties of fabrics, fibres and yarns, and the type of finishing used (16). Other variables include water vapour permeability (“breathability”) and the heat storage and release ability.

There will also be variances between individuals in terms of personal hygiene and with how long sock are worn for, and to the environment that personal socks have been exposed to (17). If socks become damp or if they are worn damp, this increases the opportunity for microbial growth (18).

Despite these variables, there is some commonality from different published reviews and the organisms isolated appear to be a combination of those that are part of the natural skin flora and those that are transient to people.

Organisms isolated in higher proportions from socks are (19 – 23):

Bacteria

  • Staphylococcus aureus,
  • Streptococcus
  • Bacillus (e.g. B. polymyxa, B. firmus, B. cereus)
  • Klebsiella pneumoniae
  • Pseudomonas aeruginosa
  • Alcaligenes faecalis
  • Enterobacter aerogenes

Fungi

  • Rhizopus
  • Aspergillus
  • Penicillium
  • Mucor
  • Candida albicans.

Of these organisms, Staphylococcus aureus occurs with the highest frequency. Hence, there is an abundance of different microorganisms recovered from socks and many of these are undesirable for the cleanroom.

Relationship between socks and foot microorganisms

Based on the above it seems reasonable to assume that the microorganisms isolated on socks are largely populated by those isolated from feet. This is borne out in medical studies examining fungal contamination (24). Here it was demonstrated that where a patient had a fungal infection, the same fungus was isolated from the patient’s sock and the environment within the sock created an environment that encouraged microbial growth and led to an increase in microbial numbers (25).

Wearing cleanroom socks

Cleanroom socks differ in terms of their quality, and some of these properties relate to the antistatic or repellence properties of the sock; other properties relate to the yearn and tensile strength. It is irrelevant, however, how good the sock is if the hands of the operator are contaminated when cleanroom socks are handled.

Given the information presented above, both human feet and personal socks are likely to be heavily contaminated with microorganisms. The act of removing personal socks will:

  1. Cause particles to be thrown around and some of these will enter the air stream as microbial carrying particles seeking a site for deposition.
  2. Cross-contaminate the hands of the operator.

While a degree of control can be introduced, such as requiring operators to sanitise their hands prior to donning cleanroom socks, this is often not practical and carries a risk of not being effective.

Instead, a better solution is to keep the personal sock on and to slip over the cleanroom sock, therefore containing a high proportion of contamination. For this to be effective, the cleanroom sock must be of the appropriate size and be strong yet lightweight to maintain both durability and comfort. An additional benefit of ‘double socking’ is to reduce the pressure impact on feet and enhancing operator comfort (26). The concept of ‘double socking’ is consistent with the approach of wearing cleanroom gowns and other measures designed to create a barrier between people and process and product components within the cleanroom.

Summary

This paper has considered the likely microbial burden on human feet – an area of ahigh concentration and species diversity – and within personal socks. These microbial concerns present a risk when operators enter cleanrooms, hence the revision in Annex 1 to wear cleanroom socks in advance of final stage changing room entry. However, given the microbial risk, if operators remove their personal socks, they will arguably present a greater risk beyond the risks presented from their personal socks when they are worn.

The points made in this paper are summarised in the following diagram:

socktosock

 

References
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