A warehouse can average 20 °C and still have corners that reach 27 °C in summer. A cold room may hold 5 °C at the control sensor and perhaps only 1 °C right in front of the evaporator. Temperature mapping makes these differences visible. It shows where products can be stored safely and where the permanent monitoring sensors belong.
What the regulations require
The EU Guidelines on Good Distribution Practice (GDP, 2013/C 343/01) are the clearest on this. They are addressed to wholesale distributors, but in practice manufacturers also use them as a reference for their storage areas. Before a storage area is first used, an initial temperature mapping exercise should be carried out under representative conditions. Monitoring sensors are placed according to the results, in the areas with the greatest fluctuations. The mapping is repeated according to the results of a risk assessment and whenever significant modifications are made to the facility or the temperature controlling equipment. For small premises of a few square metres at room temperature, a risk assessment is sufficient, and monitors are placed accordingly.
Under Part I, Chapter 3 of the EU GMP Guide (3.19), storage areas should ensure good storage conditions. Where special conditions such as temperature or humidity are required, they should be provided, checked and monitored. For the US market, 21 CFR 211.142 requires written warehousing procedures that cover, among other things, storage under appropriate conditions of temperature, humidity and light.
The WHO describes how mapping works in practice in a freely available technical supplement: Technical Supplement 8 to WHO TRS 961, Annex 9, “Temperature mapping of storage areas” (2015, published as TRS 992, Annex 5). It applies to cold rooms, freezer rooms and other temperature-controlled stores, and explicitly not to refrigerators and freezers. The ISPE Good Practice Guide “Controlled Temperature Chambers” (2nd edition, December 2021) covers temperature-controlled chambers over their whole life cycle, from specification through qualification, mapping and monitoring to decommissioning.
For medical devices, the GDP guidelines and 21 CFR 211 do not apply. Storage conditions and mapping are governed there by the quality management system under ISO 13485, for the US market 21 CFR Part 820 (QMSR), together with company SOPs. The ISPE guide is often used as good practice.
What gets mapped
Typical candidates are:
- warehouses at room temperature, for example 15 to 25 °C
- cold rooms and freezer rooms
- refrigerators and freezers
- climatic, stability and incubation chambers
Where storage areas sit within a room that is qualified anyway, such as a cleanroom or an air-conditioned production area, we integrate the mapping into the room or HVAC qualification. We apply the same principles to the choice of measuring points as in a dedicated warehouse.
Plan and protocol
Mapping starts with an approved plan. It defines:
- which area is mapped and why
- which acceptance criteria apply, derived from the products’ storage conditions
- the load condition during the study
- the study duration and recording interval
- where the measuring points are, with a rationale
- which measuring equipment is used and how it is calibrated
- how data are evaluated and how deviations are handled
For a mapping study that forms part of qualification, acceptance criteria are set before the study. Criteria defined only after the evaluation cannot be defended in an audit. The WHO also recognises studies without predefined criteria, for example to find out which products can be stored in a space or where improvements are needed. Such a characterisation study should be clearly declared as such in the plan. Incidentally, the control set points of the equipment are not acceptance criteria. What counts are the products’ storage conditions.
In practice, we only start mapping once IQ and OQ of the HVAC or refrigeration equipment are complete. Before writing the protocol, we walk the area and record its geometry, storage heights, air outlets and the positions of existing sensors.
Measuring points
Measuring points are chosen on the basis of risk. They should be where products are actually stored or will be stored, and where extremes are to be expected. That includes areas near doors and dock doors, along exterior walls and under the roof, close to supply air outlets, evaporators and heat sources, and places where racks or goods obstruct airflow. One measuring point each should sit next to the control sensor and next to the existing monitoring sensors, so that their readings can be compared with the mapping.
For warehouses and cold rooms, the WHO gives a grid with measuring points every 5 to 10 metres in length and width as a guide, and up to 20 or 30 metres in very large facilities. Up to a ceiling height of 3.6 metres, three levels are used at each grid point, for example at floor level, at 1.2 metres and at 3.0 metres. Higher rooms get additional levels, for a height of 6 metres for example 0.3, 1.8, 3.6 and 5.4 metres. Many SOPs also require a risk-based rationale for the measuring points in each room rather than simply adopting the grid. A reference logger outside the mapped area to record ambient conditions has also proven useful.
The WHO supplement does not cover refrigerators and freezers. In practice, we place measuring points there in the corners and the centre of the levels in use, plus the door and the cooling unit. We take the same approach for chambers.
Load, duration and season
An empty store behaves differently from a full one. The WHO recommends demonstrating the temperature profile both empty and in a normal loaded condition. Where real goods are not available, a representative substitute load can reproduce the airflow. This should be justified in the plan.
For warehouses at room temperature, the WHO typically gives a study duration of at least seven consecutive days, covering five working days and a weekend. For cold rooms and freezer rooms that are hardly affected by daily or seasonal variation, it gives 24 to 72 hours, longer if justified. The WHO sets no fixed time limit. If a cold room has two refrigeration units, each is mapped running on its own. Customer SOPs often require longer durations. In that case they apply.
If an area is affected by the seasons, the WHO states that at least two mapping studies may be needed: one in the warmest and one in the coldest season. For cold rooms and freezer rooms this is usually not necessary.
Depending on the risk, disturbance tests are included, such as opening the door for a defined time followed by recovery, or a simulated power failure that shows how long the limits are still met. For door openings, the WHO criterion is that the temperature returns within the limits no more than 30 minutes afterwards. The frequency and duration of the openings are defined in the plan.
Measuring equipment and calibration
Data loggers must be calibrated and traceable. For temperature loggers, the WHO specifies a three-point calibration with an error of no more than ±0.5 °C at each point. One calibration point lies below, one in the middle and one above the range being studied. The calibration should be valid (“within the current year”) and is repeated annually. Loggers with built-in batteries that are not designed to be recalibrated are the exception. All loggers get the same recording interval, between 1 and 15 minutes, and the same start time so that their readings can be correlated in time.
The measurement uncertainty is assessed against the acceptance criteria. When a reading is close to a limit, the uncertainty decides whether the result is clear-cut. If humidity is evaluated as well, the same applies to the humidity sensors.
It has proven useful to synchronise the logger clocks with the monitoring system and the building management system, and to keep an event log during the study, for example of door openings, goods movements or faults. If data are missing at a critical position, we extend or repeat the study.
Evaluation and outcome
For each measuring point, minimum, maximum and mean over the study are evaluated. These show the hot and cold spots of the area. The WHO points out that the overall trends matter, not just individual extremes. Mean values help to tell real hot and cold spots from brief outliers. Individual readings outside the range must not be disregarded, though.
The outcome of a mapping is more than a pass or fail:
- Zones unsuitable for storage, for example directly in front of the evaporator, are marked.
- The permanent monitoring sensors are placed at the critical locations, as the GDP guidelines require.
- Deviations are assessed, actions defined and repeat measurements planned where necessary.
When to map again
Under the GDP guidelines, mapping is repeated according to the results of a risk assessment and whenever the facility or the temperature control is significantly modified. The WHO gives examples of such modifications: a changed set point, changes to the refrigeration equipment, and a change in the pattern of use that increases loading or affects air circulation, such as a different rack layout. It gives three years as an example of a periodic interval. Where fixed sensors provide continuous data, a regular review of those data may be more appropriate. If the monitoring records show unexplained variability outside normal operating limits, re-mapping may be justified.
Conclusion
Temperature mapping answers two questions: is the area suitable for storage, and is the monitoring system measuring in the right place? A justified plan, risk-based measuring points, calibrated equipment and criteria set in advance produce evidence that remains traceable years later.
How room data serve as the basis for qualification is described in The room book as the basis of room qualification. Support with planning, execution and reporting is available under Qualification, and calibration of the measuring equipment under Calibration & measurement.