GXP

A Humidex For The Cleanroom: Why Temperature And Humidity Control Matters

Peer Reviewed


INTRODUCTION

Temperature and humidity control are important factors to control within the cleanroom. For pharmaceuticals and healthcare products, this relates to three areas. The first is to avoid extremes affecting the skin of the operator, and hence the ability of the cleanroom gown to contain contamination. Studies based on indoor/outdoor mass balance and receptor-based source apportionment models have demonstrated that material resuspended from surfaces as a result of human activities is an important source of indoor airborne particles (1). The second is to avoid a change in static charge that may cause particles to be pulled to surfaces, a factor influenced by changes to humidity. The third is to ensure operator comfort (which also serves to avoid the operator engaging in behaviors outside of cleanroom controls, such as engaging in activity too quickly because the operator is cold).

However, temperature and humidity are not independent factors and their interrelationship needs to be considered, in addition to airflow velocity, and the comfort levels experienced by operators themselves. For these reasons, a well-designed and maintained heating, ventilation, and air conditioning (HVAC) system  is  a  critical  part  of  pharmaceutical  cleanroom  design. By bringing together meteorological, environmental, clinical, and cleanroom data, it is possible to determine optimal parameters for the cleanroom. These are presented in the form of a novel heat index (humidex) for the cleanroom to show the ‘apparent temperature’ from the temperature and humidity relationship. This is presented within this paper.

THE RISK FACTORS

The primary risk to cleanrooms is with  abiotic factors that influence the ecology of human skin (2), especially with the rate of desquamation. This becomes more challenging the more personnel there are present within a room. Studies into variations in temperature and humidity together with room occupancy increases reveal how the total aerosol mass and bacterial genome concentration in indoor air, at the PM10 (inhalable particles, with diameters that are generally 10 micrometers and smaller) and PM2.5 (fine inhalable particles, with diameters that are generally 2.5 micrometers and smaller) size fractions, rise by nearly two orders of magnitude with each additional person present (this was in relation to airborne bacterial genome concentrations at PM10 at a temperature of 13.4±0.9°C and a relative humidity of 45±6.0%) (3).

Hence, these factors, to which we can add the rate of operator activity (4), can enhance the release of dead skin cells and this may influence the ability of the cleanroom gowns to retain them through the gown weave (which is acting like a filter). Dead skin cells are formed as new skin cells gradually push their way to the top layer. When they reach the top, they die and are "weathered" by the environment and your daily activities before they eventually fall off (5). In the ‘natural’ state the skin cycle is around four weeks, as skin regenerates itself approximately every 28 days. This means that over a 24-hour period, a typical adult will shed almost a million skin cells (or between 30,000 and 40,000 skin cells every hour; (or 12 million cells pr year, which equates to 3.6 kilograms of dead skin for the typical adult) (6). Under optimal conditions, cleanroom gowns can reduce the level of microbial carrying particles by 90% or greater (7). Within the cleanroom, where these particles (which are more often clumps of skin detritus rather than individual dells) end up depends on the design of the cleanroom air dynamics (8). However, the good design principles that have shaped the cleanroom will invariably lessen are the particle challenges increase.

There are various factors that influence the rate of skin shedding, leading to above average increases. Examples include excessive peeling a bacterial infection such as Staphylococcus aureus, which is due to the release of exotoxins (9);wound healing; pityriasis (a term capturing various types of flaking as the result of different medical conditions); sunburn (pityriasis alba); dandruff (for which there are various causes, including excessive production of skin oil in the form of sebaceous secretions (10), or from the metabolic by-products of skin microorganisms, most specifically Malassezia yeasts) (11). The factors of concern in this article are, however, temperature and humidity.

TEMPERATURE

When a person becomes ‘too hot,’ the sweat glands in the skin release more sweat. The sweat evaporates, transferring heat energy from the skin to the environment. Here the blood vessels leading to the skin capillaries become wider - they dilate - allowing more blood to flow through the skin and more heat to be lost to the environment. This is called vasodilation. This is a factor that can affect the containment ability of the cleanroom suit, as well as causing operator discomfort.

When a person becomes too cold, their skeletal muscles contract rapidly and they can shiver. These contractions need energy from respiration and some of this is released as heat, as well as causing the operator to breathe faster, which could adversely affect mask control.

HUMIDITY

Humidity is the measurement of water vapor present in the air. There are different measures of humidity: Absolute humidity describes the water content of air and is expressed in either grams per cubic meter or grams per kilogram. Relative humidity, expressed as a percentage, indicates a present state of absolute humidity relative to a maximum humidity given the same temperature. Specific humidity is the ratio of water vapor mass to total moist air parcel mass (12).

The risk factors in relation to humidity extremes is that under conditions of low humidity there is the tendency for the skin to become drier and scalier and this can contribute to increased skin shedding. Conditions of low humidity also influence the static charge within the room, for the cleanroom this could lead to airborne particles being pulled more readily to surfaces given a steady airflow state. The same effect of dry conditions could cause particles on to stick to an operator’s gloves, introducing an additional risk of contamination transfer. Further, as with low temperatures, low humidity can cause respiratory discomfort (13).

Conversely, under conditions of high humidity, there is an increase in perspiration (when matched with increase in temperature) and sweat evaporates due to excessive heat leaving moisture remaining on the skin, thereby compromising the cleanroom suit (as well as feeling oppressive for the operator). The extremities are much more sensitive to thermal discomfort from wetness than the trunk of the body (14), making the arms and hands of the cleanroom operator an additional hazard to consider. There is also the possibility of the skin becoming prone to rashes. There is also a concern that if the room is under conditions of high humidity, then microorganisms are more likely to grow and multiply, especially if condensation forms (15, 16). There is also the possibility of metal corrosion (depending on the grade of cleanroom and type of metal), water absorption, and photolithographic degradation (which could affect printed circuit boards) (17).

THE RELATIONSHIP BETWEEN TEMPERATURE AND HUMIDITY

Temperature is the average kinetic energy of the molecules, while humidity describes the amount of water vapor in the air. Vapor pressure is a function of temperature:  As molecules acquire greater kinetic energies, their increased velocities permit them to escape from the surface and get into the vapor phase. Temperature and humidity cannot be understood as separate concepts, especially when attempting to understand the feelings of comfort or discomfort on the part of the cleanroom operator. This is because, at a higher temperature, air can hold more water vapor than the same amount of air at lower temperature. With absolute humidity, the warmer the air is, the more moisture (as a specific quantity) it can hold. Conversely, the cooler the air is, the less moisture it can hold. However, this is not the case with relative humidity and the relationship is not straightforward. Moreover, variances in temperature and humidity affect the body’s response to temperature. Here the concept of ‘apparent temperature’ applies, which is the temperature equivalent perceived by humans, caused by the combined effects of air temperature, relative humidity, and airflow (18). This equivalence causes the homeostatic mechanism to react to the apparent temperature, and hence the way human skin will respond.

As an example:

  • If the air temperature reads 29˚C, but there is low humidity, the temperature will feel, from a person’s perspective, like 26 ˚C.
  • If the air temperature reads 29˚C, with 80 percent humidity, it will feel like 36˚C.

Or, in other words, high humidity and low temperatures cause the air to feel chilly as these conditions increase the conduction of heat from the body (19); and high humidity and high temperature (what Steadman (1979) calls ‘sultriness’) increases perspiration and wetness of the skin.

The human body will also react to these conditions in different ways. For instance, an increase in core or skin temperature induces peripheral vasodilation. This alters as a person becomes older. This illustrates the biological complexity around thermoregulation (20).

A further influencing factor is airflow. Airflow is a mechanism that can help with temperature and humidity control and yet, when conditions are out-of-balance, airflow can be affected with excessive variations with both temperature and humidity. While a higher airflow is generally cooling, airflow is influenced by temperature. An  increase  in  the  temperature  difference  will  increase  the  air  conditioning flow rate. This is exacerbated under conditions of low humidity (values lower than 40%), where a drop in humidity and a rise in temperature causes  an  unsteady increase in the air conditioning flow rate (21). In addition, a rise in human activity can challenge the design of the cleanroom, since vigor increases body temperature as well as the ability of the cleanroom suit to contain particles and hence excessive activity influences total particle release in clean environments (22).

OPERATOR COMFORT

Comfort in relation to the exogenous climate is difficult to determine; in terms of a definition, it is often presented as a state of thermal neutrality, which is the state achieved when the heat generated by human metabolism is allowed to dissipate and a thermal equilibrium with the surroundings is maintained. Hence, biological studies tend to indicate that ‘comfort’ is a factor of the human body’s metabolic rate (which itself varies between individuals). To this can be added the type and quantity of clothing, external temperature in relation to human body temperature (mean radiant temperature) (23), air flow, and humidity (24). Albeit this is not a universal set of criteria, for people differ as to what constitutes comfortable. Body mass is also a factor since heat dissipation is relayed to the body surface area. For instance, a tall and skinny person will have a larger surface-to-volume ratio, and they can dissipate heat more easily, and also tolerate higher temperatures ,to a greater degree than more than a person with a rounded body shape (25).

Concerns over universality has not prevented some researchers attempting to construct a comfort equation (26). In terms of the in-built environment, an ISO standard exists that provides a general set of parameters (27). For the cleanroom, temperature is normally set slightly cooler than the temperature within the office, a level that reflects the higher activity level. Spatial temperature also varies whether a person is sitting or standing, and operators in cleanrooms will primarily be standing.

WHAT IS REQUIRED FOR CLEANROOMS?

The ‘ideal’ external  temperature does not exit, as people vary in their comfort levels. There are ranges, nonetheless, based on the human basal metabolic rate (the number of calories required to keep the body functioning at rest) that are required for general comfort. These ranges vary during summer and winter, based on variations to humidity and the types of clothing that a person will be wearing.

The variations in the ranges relate to countries within which people have become acclimatized to living in, and there are also cultural and gender differences. For example, in North America the comfort level has been found to be 20–23.5°C, whereas, in Nigeria the range is 26–28°C. Furthermore, other studies have suggested that thermal comfort preferences of men and women may differ significantly, with women on average preferring higher ambient temperatures (28) (women also shed fewer particulates than men within cleanroom environments) (29). These aspects are drawn from a World Health Organization (WHO) paper looking at optimal temperature ranges that are both comfortable and not associated with health risks for healthy adults with appropriate clothing and with humidity also within an acceptable range (30) The WHO data presents the ideal humidity range as being between 30-50%, which is also matched by other studies (31). In terms of ‘too much’ humidity, the same study places the upper level for human comfort as 65% to 70%.

For cleanrooms, the WHO recommended temperature ranges are too high, given that operators are often double gowned and often engaging in activities (albeit in a slow and controlled manner). The inner cleanroom clothing provides a layer of insulating clothing which prevents heat loss (32), while this serves to keep a person warm under ideal conditions it will lead to overheating should temperatures rise outside of the target range. With cleanroom clothing, there is a trade-off between better contamination control, as delivered from tightly woven fabrics, and the concern that the greater the tightness the more thermal comfort dissatisfaction wearing such clothing produces (as demonstrated in air dispersal chamber studies) (33).

While there are no major studies that have been published in relation to cleanroom working, a study into operating theaters (where surgeons are gowned and masked) found that the optimal temperature when performing surgery was between 18 to 19°C (34).

Based on the above, what are the ideal conditions for cleanrooms? This will also be difficult to answer since any measure will need to be conditional to the statement ‘depending on the other factors involved in thermal comfort,’ in that temperature, humidity and airflow need to be considered as interrelated factors. For example, low relative humidity creates greater discomfort in terms of skin feeling itchy and dry under conditions of high air velocity (35). This means when an operator needs to intervene under a unidirectional airflow (ISO 14644 class 5 / EU GMP Grade A) the factor of humidity may need to be compensated for. Despite this complexity, we can draw out some general ranges:

  • The appropriate range for cleanroom relative humidity is in the range of 30-50% or 30-60%.
  • The appropriate range for cleanroom temperature is in the range 18-21ºC. While going lower, 16-18ºC may cause some discomfort, this is unlikely to overly influence the main risk factors. Higher temperatures will begin to affect the cleanroom gown’s ability to retain particles. Between 21 and 24oC the effects with be slower, whereas at 25oC and above we can assume relatively fast effect occurring.

This is supported by one study that showed low particle emissions within an ISO 14644 class 7/ EU Grade B cleanroom with an air temperature about 20.3°C (mean radiant temperature (about 20.8°C), a relative humidity of 55%, and an airflow velocity fluctuation of 0.2 m sec¯1. The particle emissions, across 14 points within the control room, were less than 300 counts at ≥0.5 per cubic meter, well below design requirements (36).

In terms of what happens should a cleanroom drift outside of the recommended parameters for temperature and humidity, assuming a constant airflow, it is possible to develop a humidex (humidity index) for cleanrooms. These are index numbers first developed by meteorologists to describe how the weather feels to the average person (and hence how the person’s body reacts physiologically and how they might behave psychologically). This approach looks at the twin effects of heat and humidity to produce a matrix consisting of nominally dimensionless quantities (to which the equivalent to the degree Celsius is often added).

The outcome is not linear, but instead conforms to the following pattern (Figure 1), which is itself derived from a series of equations. The equations are not reproduced here but they are accessible via the applicable reference (37).

Sandle - Figure 1 Comparison of heat index values.png

Figure 1: Comparison of heat index values (circles) with the formula approximation (curves). Source: Anderson et al (2012).

As to what might be applicable for cleanrooms, the author used the approach of Steadman (1979) and Anderson et al (2013) together with the Heat Index Calculator provided by the U.S. National Oceanic and Atmospheric Administration / U.S. National Weather Service (38). These data were used to construct a matrix (Table 1).

Table 1: Heat index (humidex) for cleanroom operators

Sandle - Table 1 - Heat Index.png

The matrix was then color coded based on the WHO data for operator comfort and the medical literature relating to skin shedding rates and sweating to produce a four-color combination. With these:

Table 2: Cleanroom heat index interpretation table

Sandle - Table 2 cleanroom heat index interpretation table.png

The above discussion on optimal values and the resultant development of the humidex is based on a series of supporting assumptions. These assumptions also underpin the original assumptions made by Steadman (1979). These are:

  • Ambient vapor pressure
  • Dimensions of a human to determine the skin's surface area
  • Temperature as an effect of radiation area of skin
  • Clothing cover and consistency of cleanroom gowns
  • Surface temperatures and vapor pressures
  • Moderate cleanroom activity (metabolic  output)
  • Constant air velocity
  • Sweating rate, assuming that sweat is uniform and not dripping from the body
  • Skin resistance to heat transfer
  • Skin resistance to moisture transfer

With Table 1, a question to consider is with how long does a temperature or humidity reading need to be out of range for? There is no straightforward answer to this because it depends on the degree that a value is outside of the range and the starting conditions. Drawing on medical data, a gradual rate of temperature decrease will cool and dry the skin and align the decrease, through vasoconstriction (as part of the body’s thermoregulatory control mechanism), with the fall in ambient conditions within 30 minutes (39). A slightly long period is required for warming, but this still remains within the hour (as based on human water bath immersion studies), from a starting point of 20 degrees and normalizing for age (40). All things being equal, it may be prudent to set a 30-minute threshold for both increases and decreases when the temperature-humidity combination moves outside of the white area.

Air Handling Units

The primary means for achieving the optimal conditions for the cleanroom is with the design and operation of the Air Handling Units (AHU) for each zone in relation to the control the temperature and relative humidity. Control is achieved, as appropriate to each area, through load variation, with the unit able to adjust for change in the sensible heat, latent heat, or for both loads, in order to create the target room total heat. This can be achieved by two strategies: first one water flow rate variation and the second one of air flow rate variation.

The generalized steps involved with temperature and humidity control are (41):

  • Outside air is filtered by pre-filter.
  • Air then passes into a pre-heater coil which addresses low outside air temperatures. The coil also provides frost protection against cold air entering the air handling system.
  • Air then goes to a mixing box, where it is mixed with a controlled quantity of re-circulated air before being conditioned in the AHU.
  • The AHU contains cooling and heating coils, humidification, and the ability to dehumidify the air. Either a heating coil or a cooling coil is operated, depending on the temperature measurement. To achieve the desired humidity, either moisture is introduced into the supply air at the AHU or elsewhere in the supply duct work (this could be via a steam injector or through the use of an atomized spray, where fine droplets are generated from clean cold water and injected into the supply air, where they evaporate rapidly). For dehumidification, an evaporator is deployed. Since the evaporator operates at a temperature below the dew point, moisture in the air condenses on the evaporator coil tubes. This moisture is collected at the bottom of the evaporator in a pan and removed by piping to a central drain (42).
  • The AHU contains the main supply fan which distributes air to the room terminal units.
  • Supply air at the required temperature and humidity is distributed into the conditioned zone by room terminal units.

DISCUSSION

This paper has presented a review of temperature and humidity factors in the cleanroom context, an area that remains under-researched in relation to cleanroom specific operations. It may also be that the control of both parameters is overlooked as part of some cleanroom designs and with their operation. The human thermoregulation system and human body responses will affect contamination levels. This is a combination of the physiological (the skin and core temperature) and the psychological (thermal sensation and thermal comfort, which can alter behaviors).

Hence, understanding and controlling cleanroom temperature and humidity is necessary to minimize particle release and to create conditions comfortable for the operator. To assist with this process a heat index has been devised for cleanrooms occupied by operators, together with recommended actions. When temperature or humidity goes out of range (into the blue or the red areas shown in Table 1), this should trigger operations to cease. For cleanroom managers not swayed by the staff welfare perspective, then contamination control factors are equally compelling. It will be unknown, for example, what will happen in relation to cleanroom suit or mask integrity (both of which have a maximum four-hour wear time under normal operational circumstances) (43). On top of this, as this paper has pointed out, the increase generation of skin cells, many of which will be microbial carrying particles, is exacerbated under non-ideal conditions.

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