A temperature chart can look perfect while relative humidity (RH) in the same room drifts out of range. If your product label, stability protocol or storage procedure names a humidity limit, a temperature-only study leaves half the question open.
Humidity mapping is a documented study of how RH is distributed through a storage space, so you know where it runs high or low and where permanent sensors should go. This guide covers when to map RH, how to design the study, how to read the data and how to move to continuous monitoring. For temperature basics in warehouses, see our guide to warehouse temperature mapping. This article stays on humidity.
What is humidity mapping?
WHO defines mapping as the “documented measurement of the temperature and/or relative humidity distribution within a storage area, including identification of hot and cold spots” (WHO Technical Supplement 8, “Temperature mapping of storage areas,” 2015, supplement to TRS 961 Annex 9, glossary). RH is part of the definition.
One caution for readers who audit documents: Supplement 8 is written mainly around temperature. Its accuracy figure (±0.5 °C), examples and analysis sections are temperature. It does not set RH limits, RH sensor accuracy or an RH sensor count. For those, rely on your product requirements, your risk assessment and a protocol you can defend.
When does relative humidity need to be mapped?
No document we reviewed gives a single rule that says “map humidity whenever X.” The practical test is whether humidity is a controlled parameter for the space. Common cases:
- Products with a humidity statement on the label or in the registration. 21 CFR 211.142(b) requires written warehousing procedures covering storage “under appropriate conditions of temperature, humidity, and light.” The section does not use the word mapping, but a study is a common way to show those conditions hold.
- Controlled-RH storage rooms where a humidity range is written into the procedure.
- Stability chambers. ICH Q1A(R2) lists storage conditions that pair temperature with RH, such as 25 °C ± 2 °C / 60% RH ± 5% RH (long term), 30 °C ± 2 °C / 65% RH ± 5% RH (intermediate) and 40 °C ± 2 °C / 75% RH ± 5% RH (accelerated). 21 CFR 211.166(a)(2) requires a written stability program that includes storage conditions for samples. See mapping stability chambers and when to re-map.
- Cleanrooms and controlled manufacturing areas. 21 CFR 211.46(b) requires equipment for adequate control over humidity and temperature, among other conditions.
If none of these apply, a documented risk assessment saying humidity is not a controlled parameter is a reasonable outcome. Write it down either way.
How is humidity mapping different from temperature mapping?
The method is the same: calibrated loggers, a written protocol, a defined period, pre-set criteria and a report.
| Point | Temperature mapping | Humidity mapping |
|---|---|---|
| What varies by location | Air temperature, shaped by supply air, heat sources, doors and stratification | RH, which depends on moisture content and local temperature, so a cold spot often reads as a wet spot |
| Sensor response | Fast in air | Usually slower, so allow time to stabilise |
| Common drivers | HVAC, sun load, doors, racking | Dehumidification, outdoor weather, door openings, wet floors, open water, process moisture |
| Summary statistics | Min, max, mean; MKT sometimes used | Min, max, mean; MKT does not apply |
The sensor and driver rows reflect common practice, not a quoted rule. Because RH and temperature are linked, map them together.
How do you design a humidity mapping protocol?
Write and approve the protocol before collecting data. WHO lists its contents in section 2.2: approval and change control history, description and rationale, scope, objectives, methodology and a report template. For RH, work through the items below.
Sensor count and placement
No source we verified sets a fixed RH sensor count. WHO’s temperature guidance for warehouses is a useful frame: loggers on a grid every 5 to 10 metres across length and width (up to 20 or 30 metres in very large facilities), stacked at high, medium and low levels where ceilings are 3.6 metres or less, using a risk-based approach (Supplement 8, section 2.2.6, Step 5). A common approach is to reuse that three-dimensional grid for RH and add positions where moisture risk is highest: dock and personnel doors, supply and return air, humidifier or dehumidifier outlets, exterior walls, floor level and dead-air corners. Put one logger beside the existing control sensor to compare it with the rest of the space, and record why you chose the count and positions.
Duration and sampling interval
WHO says there is no formal time limit. For warehouses and ambient storage it gives a typical minimum of seven consecutive days, including five working days and two weekend days; for cold and freezer rooms, 24 to 72 hours (section 2.2.6, Step 9). It asks for the same recording interval on every device, typically 1 to 15 minutes (section 2.2.6, Step 7). These figures are written for temperature, so justify any choice for RH. Log door openings and power or HVAC events.
Empty or loaded, and worst-case conditions
WHO says mapping should show the profile when the area is empty and in a normal loaded condition (section 1.1). Load matters for humidity: cartons, pallet wrap and dense racking slow air movement and can create pockets. Our post on empty versus loaded mapping explains the reasoning, which carries over to RH. Also decide which worst-case conditions to include, such as a defined door-open challenge or a humid time of year.
Seasonal considerations
WHO says that where storage is affected by seasonal temperature variation, “at least two temperature-mapping studies may be needed,” typically one in the warmest and one in the coldest season (section 2.2.4). That sentence is about temperature, and it says “may.” The EU GDP Guidelines (2013/C 343/01, section 3.2.1) say the mapping exercise should be repeated according to the results of a risk assessment or after significant modifications. Outdoor humidity changes with the weather too, so it is reasonable to put RH in the same documented decision. For the temperature side, see what happens when mapping misses seasonal extremes.
What sensors does a humidity mapping study need?
- Combined temperature and RH loggers of one device type per study. WHO makes the one-type point for temperature (section 2.2.6, Step 1).
- Calibration traceable to national standards and current at the time of the study. WHO asks for NIST-traceable three-point calibration of temperature loggers, with certificates in the report. Its ±0.5 °C figure is a temperature figure and does not carry over to RH.
- RH calibration points that span your range. By analogy with WHO’s temperature rule (one point below, one in the middle, one above the range), calibrate across the limits you will test against.
- A stated RH accuracy. Put it in the protocol and compare it with the acceptance band. If sensor uncertainty is a large share of the band, a pass is hard to defend.
- A post-study check. Many teams re-verify loggers afterwards to know of any drift. This is common practice, not a rule we found.
Qualified Controls wireless sensors measure temperature and humidity and are supplied calibrated, with certificates for the report. See also calibrated sensors for audit-ready labs.
What acceptance criteria should you set for RH?
Define limits before the study. They come from the product label, registration, stability protocol or storage specification, not from what the room happens to achieve. For chambers, the ICH Q1A(R2) conditions above are the reference point; for a warehouse, use the most sensitive product stored.
No standard we reviewed gives an RH figure that applies to every pharmaceutical store, so do not borrow one from a vendor page. If you test door openings, state the allowed recovery time.
How do you analyze humidity mapping data?
Follow the structure WHO describes for temperature (section 2.4) and apply it to RH and temperature together.
- Check data integrity. Confirm start times, intervals, serial numbers, calibration certificates and gaps.
- Calculate min, max and mean RH at each position and compare each with the limits.
- Find the wet and dry spots. As WHO says of hot and cold spots, they show where monitoring sensors should preferentially be located (section 2.4.2).
- Read RH with temperature. A high-RH location is often a cold one, which tells you whether to fix airflow, the dehumidifier or the load layout.
- Look at time as well as position. Cycling, recovery after door openings and weekday versus weekend differences.
- Do not use MKT for RH. ICH Q1A(R2) defines MKT as a single derived temperature giving the same thermal challenge as a range of temperatures, based on the Arrhenius equation. There is no equivalent for humidity, so judge RH against the limit at each location and the duration of any excursion.
A mean RH inside the range does not prove every position was acceptable.
What should the humidity mapping report contain?
WHO’s template (section 2.2.7) is a workable structure: introduction and objectives, results in protocol order including deviations, conclusions and recommendations (including whether the area is acceptable for operation), and annexes with the site survey and logger positions, raw data and graphs for each logger, the analysis, deviation reports with corrective actions and calibration certificates for every logger. For RH, add the humidifier and dehumidifier set points in force, the weather during the study and the temperature data beside each RH result. WHO says the report should be audited and peer reviewed by a competent independent person (section 2.4.5).
When should humidity be mapped again?
WHO says mapping should be carried out whenever significant modifications are made to the store, and that re-mapping may be justified when analysis of temperature and/or humidity monitoring records shows unexplained variability outside normal operating limits. It adds that periodic mapping, for example every three years, may be required depending on the routine monitoring strategy (section 1.1). Typical RH triggers: a change to dehumidification or HVAC, a new layout or load pattern, or a changed product humidity limit.
From one-time study to continuous RH monitoring
A mapping study is a snapshot. Humidity moves with weather, door traffic and equipment condition, so continuous monitoring is what shows what happens between studies. Use the map to choose permanent sensor locations: the wettest and driest positions found, plus a representative position for the main storage area. Record the link between map and sensors in the monitoring plan.
The Qualified Controls monitoring platform collects readings from Qualified Controls wireless sensors around the clock, alerts the right people when RH or temperature crosses a limit and keeps the history for trending and reports that support GDP and GMP expectations. Set alert and action limits from your acceptance criteria, and allow for slower RH response to limit nuisance alarms. See stability chamber monitoring during HVAC failures and beyond lab temperature.
Need help with the study itself? Qualified Controls provides mapping and validation services.
Humidity mapping checklist
- Risk assessment records whether RH is controlled; protocol approved before the study
- Limits taken from the label, registration or stability protocol
- Site survey and sensor positions, including 3D levels and high-risk spots
- Combined temperature and RH loggers, one type, calibration current
- RH accuracy stated and compared with the acceptance band
- Duration, interval, empty or loaded state and worst case justified
- Seasonal decision documented; door, power and HVAC events logged
- Min, max and mean RH per position, wet and dry spots read against temperature
- Report reviewed, with deviations and corrective actions
- Permanent sensor locations and alarm limits set from the results
- Re-mapping triggers written into the procedure
Humidity mapping FAQ
What is humidity mapping?
Humidity mapping is a documented study that measures how relative humidity is distributed through a storage area or chamber using calibrated loggers over a defined period. It finds the wettest and driest locations and shows where permanent sensors should go.
When do I need to map relative humidity?
Map RH when humidity is a controlled parameter: products with a humidity limit, controlled-RH storage, stability chambers and cleanrooms. We found no blanket rule, so document the decision in a risk assessment.
How long should a humidity mapping study run?
There is no formal time limit. WHO Supplement 8 (section 2.2.6, Step 9) gives a typical minimum of seven consecutive days for warehouses and 24 to 72 hours for cold and freezer rooms. These are temperature figures, so justify your RH duration.
Does mean kinetic temperature apply to humidity?
No. Mean kinetic temperature is based on temperature and the Arrhenius equation, as ICH Q1A(R2) defines it. There is no RH equivalent, so compare RH with the limit at each location and review how long any excursion lasted.
How many sensors does a humidity mapping study need?
No verified standard sets a fixed number. A common approach is the WHO temperature grid for warehouses (loggers every 5 to 10 metres, stacked by height) plus extra positions at doors, air outlets, exterior walls and dead-air areas.
Do I need to map humidity in summer and winter?
Not by any rule we found. WHO says two temperature-mapping studies “may be needed” where seasonal variation affects storage, and EU GDP section 3.2.1 says to repeat mapping according to a risk assessment. Outdoor humidity changes with the season, so record your decision and rationale.
