Sliding doors moment: Unlocking an underappreciated cleanroom control factor

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Oct 8
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Technical Review Article | Open Access | Published 8th October 2025

Sliding doors moment: Unlocking an underappreciated cleanroom control factor

Tim Sandle, | EJPPS | 303 (2025)| https://doi.org/10.37521/ejpps30209

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Abstract

A multitude of measures are used to minimise contamination ingress into ‘cleaner’ cleanrooms, as presented by less clean areas. Airlocks serve a significant role in achieving this effect. However, for cleanrooms interfacing corridors, particularly in aseptic facilities, there are several vulnerabilities which need to be considered. These include the behavioural elements of personnel (walking speed and motion, numbers of people and so on) together with design factors including the type of door, opening speed, door opening direction and airflow rates. This paper weighs up these collective factors and presents a set of optimal assumptions to help minimise contamination through door openings.

Introduction

Cleanrooms, built to appropriate specifications, in the static state are generally reliable at achieving their objectives of low particulate air and with the minimisation of particles settling out onto surfaces through deposition. Cleanrooms in the ‘in use’ state are, on the other hand, subject to what Yang and colleagues refer to as “disturbance factors”¹. One such factor is the opening and closing of doors and personnel access.

Differential pressures (typically 5–20 Pa, to provide sufficient magnitude and stability) are applied between the cleanroom and adjacent zones, such as corridors to process rooms, or airlocks separating cleanrooms of different grades. This pressure range prevents, under normal circumstances, contaminated air from moving into the higher graded cleanroom (as might occur through a slit within the door between the two adjacent spaces) as well as to prevent the reversal of the airflow direction of the air leaving the cleaner area.

When a door is opened, however, there is a possibility that the differential pressure drops and may reach neutral almost immediately. Consequently, the aerodynamic ‘defensive’ barrier for airborne particles could become significantly weakened. Nevertheless, with a large enough supply-exhaust flow rate differential, a directional airflow can potentially be maintained across an open doorway. This helps to minimise the hazard represented by airborne particulates from entering the less clean area.

Even when air control is maintained in this manner, the risk of particle ingress can become elevated when a ‘dynamic disturbance’ occurs. This will be commonplace since it includes human movement. Most doors are opened and then closed to enable personnel to enter or exit, or to pass something through, and door motion induces two-way flow across the doorway. The passage of personnel through the doorway results in an instantaneous rise in the particle concentration in the cleanroom area near the doorway.

Disturbance is most significant as a person transitions from the lower grade to a higher grade area. This activity can induce contaminated air to enter the cleaner area. This occurs because human walking disturbs the local velocity field², a phenomenon termed wake formation³. The actual level of risk is influenced by secondary factors such as the rate of door opening, door type, temperature differences between the two areas, and the speed of passage through the door.

Although many facilities will place restrictions on the length of time that a door can remain open, this is often an arbitrarily set figure, invariably geared around the typical task time. The objective is to minimise the loss of pressure to the room and unwanted air movement.

However, is there more that can be done to evaluate and understand the risk of particle transfer from one cleanroom to another? This review article looks at the seemingly simple, and for many even seemingly uninteresting, concept of cleanroom door design and the impact of opening them and with personnel or materials transitioning through them. Such activities present an important consideration when considering contamination control in relation to cleanroom control. Moreover, the considerations are not only of potential interest to the pharmaceutical sector, but they are also applicable to healthcare operating theatres.

Cleanrooms, airlocks, and pressure control

Cleanrooms are designed with a number of contamination control features. Among these is pressure; a cleanroom is maintained at a higher pressure to its surrounding areas. This can sometimes be configured between rooms of the same grade, such as from a process room to a corridor; however, this differential is of a greater importance where the two adjacent areas are of a different grade (such as an EU GMP Grade B room interfacing with an EU GMP Grade C area). When the door is opened, the aim of the pressure differential is the outwards flow of air to be sufficient to prevent particles from entering. In current regulations, the air pressure differential should be a minimum of 10 Pascals. This is above the level where ingress will occur unaided, that is, without a physical force moving against the pressure flow and into the cleaner area, which according to some cleanroom experts is at 3- 4Pa, albeit only for a brief period.⁴. If there is a pressure drop upon opening, followed by a recovery period upon doors closing, then it will take a proportionately shorter time under conditions of a higher positive pressure differential compared with a lower positive pressure differential. Even under these conditions, air mixing can sometimes occur⁵.

Hence, to minimise the risk of pressure drop and the force of air from two sources mixing, facilities will deploy airlocks where two different grades of cleanroom meet. When operated correctly, through interlocking doors, airlocks help to maintain containment of contamination and allow for its dispersal and removal.

An airlock consists of a room or chamber with two airtight doors, arranged in series and which do not open simultaneously. When both doors are closed, a physical barrier is in place. When one door is opened, air will flow from the cleanest area into the airlock through the control of air pressure differentials. Airlocks facilitate the controlled movement of materials or people between different cleanroom zones (special airlocks, designated changing rooms, are used for the passage of people). When passing through one of two doors of the airlock, personnel should only be able to open the second door once the first door has securely closed, through an interlock system, and after a suitable period has elapsed to eliminate any particles transferred from the lower grade area. While a proportion of these particles may settle, the majority should eventually be removed through extracts through the air exchange operation. Hazards to materials to be transferred to a cleaner area can be further reduced through disinfection.

There are three types of airlocks:

• Cascade airlock: This maintains a midpoint pressure between a high-pressure area (such as an EU GMP Grade B cleanroom) and a low-pressure area (such as a corridor).

• Bubble airlock: This maintains a higher pressure inside the airlock than the adjoining spaces, pushing air and contaminants outward. The bubble type of airlock system is deployed where the product needs protection, and the person external to the cleanroom requires protection from the product.

• Sink airlock: This maintains a lower pressure inside the airlock, drawing air inward from adjacent areas to trap particles. This can be used where two adjacent areas of the same grade meet or where hazardous substances are managed.

The most common type of airlock is the cascade airlock, although this will depend on the nature of the facility and its operation.

For rooms of the same grade, there is often the need to maintain a pressure differential in order to help protect a more critical operation. An example might be between a corridor and a room housing aseptic filling equipment. However, the passage of personnel and equipment through the door will inevitably lead to particle entrainment.

Particularly in situations where there is no airlock, yet for which a pressure differential is desirable, the design and operation of cleanroom doors influence the degree to which particles might be pulled into the cleaner area. At times, this is a neglected area of cleanroom design.

Lowering contamination ingress through door design

The main part of this article focuses on cleanroom door design and the varied factors to be considered for reducing contamination ingress in the form of particle entrainment or diffusion⁶. This is approached through a series of considerations in answer to the seemingly simple question: What happens when a door opens?

To answer this, we need to consider factors like:

The speed of movement.
The type of door.
Speed of door closure.
How long the door remains open.
The ventilation rate.
The number of personnel moving through.

Speed of movement

The speed at which a person moves through a door from one cleanroom to another, or from an airlock, influences the likelihood of particle transfer, with the likelihood increasing the faster the rate at which a person moves. By undertaking experiments to demonstrate large eddy simulations, researchers have demonstrated the effect of human motion on the particle transport for both a room-to-room and a room-to-corridor configuration. Here, the number of transported particles is influenced by the walking speed together with the distance of the person from the doorway⁷ and the size of the opening, larger openings allowing for greater airflow and thus more significant heat and mass transfer⁸. Looking at this from the perspective of particle levels, Zhou and colleagues found that, on average, the particle intrusion ratio with walking speed 0.60 m/s is less than that with walking speed 0.80 m/s and 1.00 m/s by 30% and 42%, respectively⁹. Contamination transfer does not immediately stop when the operators stop on entering the room, or even if they slow down, for the transport of contaminants in the direction of movement continues for several seconds due to inertia, even when the subject stops.

Temperature, including the temperature of the person moving through a door, is also a factor when design is considered. Temperature difference between the spaces, opening size, and the thermal properties of the partition all influence the rate of heat transfer. If the heat transfer gradient is towards the cleaner room (i.e. the cleaner room is cooler), as heat moves in the air current, the likelihood of particle transfer similarly rises. Higher temperature is important in relation to people generated particles due to the presence of thermal plumes¹⁰. This effect is more prominent as walking increases up to a threshold of around 0.6 m/s, after which wake flow dominates over the thermal flows. Where thermal plumes remain influential, as induced by the body heat, this can contribute to particle transfer. For example, as an individual increases walking speed from 0.2 to 0.4 m/s, the contribution to temperature similarly doubles¹¹. The contribution of personnel is something important to factor into computational fluid dynamics when cleanrooms are modelled for their airflow patterns.

Wake flow refers to the disturbed, often turbulent, region of fluid that trails behind a moving object. When walking, the wake is characterised by a region of recirculating air and vortices, particularly noticeable behind the legs. As a person walks at a faster pace, the thermal plume (the warm air rising from the body) is swept backwards, creating a wake. The wake can affect how particles are carried and dispersed in the air, as shown through the impact of source motion on the contaminant distribution in cleanrooms. The faster a person walks, the greater the wake motion. As a person nears the doorway, this causes vertical mixing of airflows, and this can serve to carry particles greater distances (the region affected by the flow field expands)¹²; walking triggers both the horizontal migration of particles and the downward transmission of aerosols and larger particles¹³. The most significant shedding region is behind the leg, making this a potential target site for exit suit gown monitoring. In addition, the spacing and pacing of personnel becomes important. Personnel entering critical areas should not immediately follow one another, to avoid the leg level shedding vortex transferring particles from one individual to another; neither should personnel move through an opening together side by side, as this will enhance the particle concentration released through horizontal dispersion.

That the marginal contribution of another person in any given situation increases the rate of particles should come as no surprise. It has previously been shown that, in a Grade B cleanroom, the particle concentrations measured will typically increase by approximately 2,000 particle counts of a diameter of ≥0.5 µm per m³ with the addition of each additional person.¹⁴. This level, of course, depends on the quality of the gown and an assumption that the person is stationary. In this case, the 2000 value relates to double cleanroom gowning as per an aseptic processing area, anything less builds upon the fact that human particle emission rates are 50,000–180,000 particles per minute per person without wearing any cleanroom gowns and instead wearing ‘everyday clothing’¹⁵.

Walking rapidly can also cause particles that have settled on surfaces to be resuspended into the air, increasing the concentration of airborne particles (the phenomenon of walking-induced particle resuspension). Both airflow and lower relative humidity can weaken adhesion, potentially increasing resuspension.

Door closure speed

Cleanroom doors should not be opened or closed rapidly, for this can increase the amount of air taken in from the lower grade of the cleanroom. To counter this effect, it is useful for doors to be equipped with a closing mechanism that controls the speed at which a door can close¹⁶. Automated, touch-free mechanisms provide additional contamination control advantages since the operator does not need to physically touch the door itself hence reducing the prospect of surface-to-surface or human-to-surface contamination transfer.

Inwards or outwards

Which way should a door open? Outwards to the lower graded area or inwards into the cleaner area? Inwards appears to be optimal (that is, doors should open in the direction of higher pressure to minimise the risk of contamination). Studies suggest that moving against the opening direction of the door (i.e. when entering the cleaner area), the passage amplifies the doorway flows, resulting in a higher air volume transfer across the doorway, compared with either the door opening outwards or when exiting from the room¹⁷. Nonetheless, it is worth noting that some other studies suggest this difference is minimal and, should it be a concern, the effect can be significantly lowered and cancelled out by increasing the air exchange rate¹⁸.

Door type

The type of door can also influence the risk of particle transference¹⁹. One study found double-hinged doors resulted in the greatest risk of particle leakage into or out of the room, this is followed by single-hinged doors (which reduce the level of particle transfer) and then by double-sliding and single-sliding doors, each lowering particle transfer lower still²⁰. This decreasing level of particle transfer is due to, for each door type in turn, a lower induced airflow (the secondary airflow that is drawn into a system due to the primary airflow through the act of movement)²¹. The most problematic type of door is a rotating door, to the extent that the impact of a rotating door on particles was approximately six times greater compared with a sliding door. This is not universally accepted, for some researchers have discerned no significant differences between hinged and sliding doors²². Nonetheless, if we take an objective overview of research, the balance is in favour of sliding doors since the hinged door motion-generated airflow is more pronounced compared to the sliding door²³.²⁴.

Despite the seeming advantages of sliding doors²⁵, it must be noted that hinged doors are common in the pharmaceutical cleanrooms due to some practical factors such as space restriction, airtightness, and generation of particles from door components. Hence, we need to consider all of the design elements holistically. Some practitioners recommend the use of curtains across doorways to help minimise contamination ingress²⁶; however, curtains are contamination traps and often prove notoriously difficult to clean.

Length of opening time

It will not be a surprise that the length of time that a door is open is a rising problem for particle control: longer door-opening duration and lower air change rate produce more risk, and, conversely, once a door is closed, the particle concentration begins to abate²⁷. Many studies have established that the extent of leakage caused by personnel entry was affected by the door swing time (how long the door is open for) ²⁸, indicating this is greatest as air is drawn in during opening (most of it from the front face of the door) or when air is pushed through during closing (in-leakage air is pushed and compressed by the front face and flows round the door edges to the back face). In these studies, the doors were open for between 3 and 8 seconds, and the impact was demonstrated using a tracer gas. The rate of in-leakage was relatively constant, averaging at 0.6m³.

The longer that a door is open for is additionally affected by the temperature differential²⁹; a marked temperature gradient substantially increases the total air volume migration during door operation³⁰. Plus, we need to consider the force of any forward motion,

Door opening speed.

Related to ‘swing’ is the rate at which a door opens. This is impacted by the air density. At a density difference of zero, the total exchange volume is proportional to the door swing speed³¹. This means the faster a door opens, the greater the potential to draw in air. Therefore, having a door that opens relatively slowly is advantageous.

Variables

There are, of course, many variables that can be added to the likelihood of contamination transfer. One important consideration arises from the fact that contaminant dispersion is highly dependent on surrounding airflow patterns, especially when moving through a door. These patterns are influenced by ³²:

• Room dimensions: here we need to acknowledge this as ‘potentially’, for there seems to be no clear consensus on how the room sizes affect the door motion-induced flows and air volume migration through the doorway.

• Ventilation patterns,

• Air speed,

• Air direction,

• Air density (which is often a product of a temperature difference)³³,

• Air temperature.

Considering some of these:

Air temperature

Temperature has featured in several of the above sections. To further consider relative heat, it is not always a case that a risk is presented when the cleaner room is cooler than the less clean room, for where a temperature gradient exists between the warm air inside the ‘cleaner’ room and the cooler air outside in the less clean air, this can also create a ‘two-way buoyancy flow’. This effect can encourage the warm air to rise as it flows out of the cleaner room, while cooler air from the outside area flows in at floor level³⁴. Given that microbial carrying particles invariably gravitate to the floor, this effect can be problematic (resuspension estimates increase with particle size in the range of 0.7–10 μm)³⁵.

Ventilation

The rate of ventilation through a doorway, from the cleaner to the less clean area, is a key factor. To illustrate this, UK NHS Health Technical Memorandum 03–01 recommends 470 L/s (0.29 m/s) airflow rate through an open doorway in steady state conditions from a clean air area (used for aseptic operations) to surrounding areas³⁶, to ensure that reverse air flow does not occur (assuming no significant temperature difference). This represents a useful yardstick, but what happens when people pass through?

To evaluate doors opening airflows together with personnel transport, scientists discovered that increasing the flow differential (i.e. directional airflow) from approximately 30 L/s (0.01 m/s) to 100 L/s (0.05 m/s) through a doorway reduced the air volume migration from a cleaner room to the surrounding room from 1.55 m³ to 1.20 m³ on average. The researchers went on to estimate, assuming a linear dependence, that 360 L/s (0.17 m/s) airflow is needed to reduce the air volume migration close to zero ³⁷.

While interesting, this did not account for the speed at which personnel moved through the doors. In a separate inquiry, researchers evaluated the particle transmission impact of a person entering from an airlock to a cleanroom through a doorway. The study included variations to the doorway airflow rates, 210, 400 and 580 L/s, and personnel walking speeds of 0.5 and 1.0 m/s. During these scenarios, particles were elevated in the airlock. This found³⁸:

Particle concentration in the cleanroom was increased by the entrance of the person (which supports the information presented above).
The lower the airflow rate, the greater the concentration of particles likely to enter the cleanroom.
The faster a person moved, the greater the concentration of particles likely to enter the cleanroom.
Faster airflows were, if correctly configured, able to counter-balance the speed of personnel movement.

The conclusion was that optimising the airflow at the door opening created an aerodynamic barrier to control of dynamic disturbance.

Evaluation

How can we evaluate if we are in control? While physical measurements and trace gas studies, along with techniques like computational fluid dynamics (CFD), can provide solid data, straightforward assessments like airflow visualisation studies and particle counting can provide more immediate assessments using equipment that is readily available to the cleanroom manager.

Table 1 below summarises the information presented in the main part of this paper and sets out ‘points for consideration’ for the design and subsequent control of cleanrooms.

Table 1: Contamination control factors related to cleanroom door openings.

Factor	Qualitative observation	Quantitative measure
Behaviours
Speed of movement	The faster a person moves through a doorway, the more likely particle transfer is to occur due to the wake effect.	Increasing walking pace from 0.6 m/s to 0.8 m/s increases particle transfer by 30%; increasing walking pace from 0.6 m/s 10 1.0 m/s increases particle transfer by 42%.
	Rapid walking causes larger particles settled on the floor to be resuspended. This can be enhanced by warm air rising from the floor.	N/A
	The further away a person is when the door opens leads to increased motion, which can also lead to an increased chance of particle ingress.	N/A
	Transfer continues for several seconds as a person moves through the room.	N/A
	The greatest concentration of particles is found around the legs of a person.	N/A
	Particles released from a person take a horizontal and downward path, based on the size and mass of the particles with larger particles moving downwards through gravitational settling and other physical forces.	N/A
	Individuals should not enter through a doorway side-by-side, as this amplifies horizontal particle distribution.	N/A
	Individuals should keep a distance between themselves as they move through the doorway to avoid particle transfer and increase the concentration of particles carried by the person behind.	Each person, when stationary, increases the particle concentration in each space by 2000 particle counts per cubic metre (≥0.5 µm).
Design
Size of the doorway opening	Larger openings tend to allow greater airflow, presenting a lower risk of particle ingress compared with narrower openings.	N/A
Temperature differences	Temperature differences The larger the temperature difference between one room and another, the greater the influence of the rate of heat transfer. Heat transfer towards the cleaner room increases the likelihood of simultaneous particle transfer. Personnel can contribute to this by walking faster.	Faster walking, from 0.2 to 0.4 m/s, doubles the influence of human contribution to the temperature gradient.
Door opening speed	The faster a door opens, the greater the potential to draw in air.	N/A
Door closure speed	Cleanroom doors should not be opened or closed rapidly, as this can increase the amount of air taken in from the lower grade of the cleanroom. It should be noted that all door closing pushes some particles into the cleanroom	N/A
Door opening direction	If one room has a positive pressure and another has a negative pressure, the door should open into the positive pressure room to minimise contamination transfer.	N/A
The time the door is open for	The longer the time that a door is open, the greater the potential risk	The relative inward flow is 0.6 m/s for each second a door is open, depending on temperature and the rate of forward motion.
Touch-free doors	Doors that open automatically remove the need for operators to touch door opening mechanisms, and hence, a source of surface-to-surface contamination transfer.	N/A
Door design	Door design The following hierarchy of increasing risk applies: • Single sliding doors (lowest risk) • Double sliding doors • Single-hinged doors • Double-hinged doors, Rotating door (greatest risk) N/A	N/A
Airflow through the door opening	The rate of ventilation through a doorway, from the cleaner to the less clean area, affects particle transfer, with higher outward airflows being preferred.	A rate of 470 L/s (0.29 m/s) through an open doorway from the clean air area to surrounding areas can minimise particle ingress. As a minimum, 360 L/s (0.17 m/s) airflow is needed to reduce the air volume migration close to zero.

Summary

What to make of these studies? It would appear that to lower the risk of contamination transfer from a dirtier to a cleaner area (drawing on Table 1), reveals:

1. Sliding doors are superior to hinged doors, as a consequence of the door's motion.

2. Temperature gradients should be minimal.

3. Doors should only be kept open for short periods (such as 8 to 10 seconds).

4. Operator movement should be controlled and steady.

5. High ventilation rates are required.

In addition, the use of airlocks significantly helps to mitigate the transfer problem. However, we must consider that doors from corridors to process rooms (including aseptic processing areas) rarely have airlocks.

Much of the information presented in this paper may seem obvious to more experienced readers (such as the number of people entering the room and extension of the door opening time may cause the invasion of more particles to the cleanroom with higher cleanliness), yet it remains interesting to position what we know about some of the research conducted and it remains important to assess whether our physical controls are continuing to work within their design specifications and the extent to which we know this to be the case through periodic reviews of the data. Moreover, for those less experienced in contamination control, it is hoped some of the findings will prove to be of interest, if nothing else, when considering where to place room particle counters in terms of the relative risk of personnel access.

The behaviour of people matters in cleanrooms. Despite the maintenance of pressure gradients across a doorway with high air change rates, particles in one room can spread into the other 'cleaner' room when the door is open, especially as the result of contaminant entrainment in the wake induced by human motion is the dominant transport mechanism. This is enhanced by physical-mechanical factors throughout the door opening and closing motion, the door sweeping effect in combination with the ventilation system dominates two-way airflow and contaminant exchange. Hence, both design and personnel behaviour control remain important variables for cleanroom contamination control.

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