What an air sampling device does and how counts are readAn air sampling device — a microbiological air sampler — draws a measured volume of air across a nutrient agar plate so that any airborne micro-organisms are captured, grown into visible colonies, and counted to give a result in colony-forming units per cubic metre (CFU/m³). Reading the count is the final step: after incubation the colonies on the plate are counted, corrected, and divided by the volume of air sampled to express contamination per cubic metre. That single figure is how a cleanroom or theatre proves its air meets its microbiological limit.For a lab, pharmacy or cleanroom buyer, the sampler is a monitoring instrument that must be calibrated, validated and matched to the environment it polices. This guide explains the capture principle, how counts become CFU/m³, and what to specify.Impaction: capturing microbes onto agarActive air samplers work by impaction. A pump pulls air through a perforated head at a known flow rate — commonly around 100 litres per minute — and the air jets accelerate particles so they strike the surface of an agar plate placed beneath the head. Micro-organisms too heavy to follow the airflow around the plate impact onto the agar and stick. The physics is characterised by a cut-off diameter (d50), the particle size the sampler collects with 50% efficiency; a well-designed head captures the small particle sizes that carry viable organisms. Air then exhausts, having deposited its biological load on the medium.From colonies to CFU per cubic metreTo turn a plate into a result you need both a count and a volume. Sampling 1,000 litres gives one cubic metre, so a sampler set to draw 1,000 litres and yielding, say, 12 colonies reports 12 CFU/m³ before correction. The correction matters: on a sieve impactor, two or more organisms can enter through the same hole and grow as one colony, so a statistical positive-hole (Feller) correction is applied using the head's hole count, raising the true count especially at higher contamination. Only after that correction and the volume division is the CFU/m³ figure valid.Active samplers versus settle platesImpaction samplers are active — they force a known volume of air onto the medium and give a volumetric CFU/m³ result. Settle plates are passive: a 90 mm agar plate is left open, often for up to four hours, and organisms sediment onto it under gravity, giving a result per plate per exposure time rather than per volume of air. Settle plates measure deposition onto a surface and are cheap and simple, but they cannot report CFU/m³. Cleanroom monitoring typically uses both — active sampling for the volumetric figure and settle plates for surface-deposition trend.Sieve, slit and other impactor typesThe common design is the sieve impactor, whose head is perforated with many precision holes creating an array of air jets onto a standard agar plate — simple, robust and widely used for routine monitoring. A slit-to-agar sampler draws air through a narrow slit onto a slowly rotating agar plate, so the colonies are spread around the plate in time order, allowing a contamination event to be located within the sampling period. Other formats include gelatine-membrane and centrifugal samplers. Each trades simplicity, time resolution and gentleness on the organism differently.Cleanroom grades and the limits that applyIn pharmaceutical manufacturing the reference is EU GMP Annex 1, which grades cleanrooms A to D and sets recommended airborne viable limits: Grade A at less than 1 CFU/m³, Grade B at 10, Grade C at 100 and Grade D at 200 CFU/m³. Grade A — the critical zone for aseptic filling — effectively means no growth is expected. Physical particle cleanliness is classified separately under the ISO 14644 family, while viable monitoring and biocontamination control follow ISO 14698. The grade dictates how much air you must sample: reaching a Grade A limit of under 1 CFU/m³ requires sampling a full cubic metre.Sample points, timing and grade of environmentWhere and when you sample is as defined as how. In an aseptic pharmacy or manufacturing suite, sample points are fixed at the locations most likely to reveal a problem — close to the critical filling zone, at working heights, and at room-return positions — and monitored on a schedule that intensifies with the grade of the area and whether it is at rest or in operation. Grade A zones are monitored during every critical operation; lower grades are checked periodically. Trending the results over time matters more than any single reading: a stable low count confirms control, while a rising trend flags a developing problem before a limit is ever breached. Fixed continuous monitoring exists for the most critical zones, but for routine environmental rounds a validated portable impaction sampler that is quick to set up at each point remains the standard tool. Air sampling is only one leg of environmental monitoring, which also uses surface sampling swabs on contact surfaces and water testing kits on the water system.Incubation, media and reading the platesThe plate defines what you can detect. General bacterial monitoring uses tryptic soy agar (TSA), while fungal monitoring uses Sabouraud dextrose agar; incubation regimes typically run TSA warm for bacteria and a cooler incubation for moulds and yeasts, each for a set number of days. After incubation an operator counts every discrete colony, applies the positive-hole correction, and records the CFU/m³. Contact and settle plates are read the same way. Consistent media lots, incubation temperatures and reader training keep results comparable over time, and the media and rinse solutions in turn depend on reagent-grade water from laboratory water purifiers.Calibration and flow-rate verificationA volumetric result is only as trustworthy as the flow rate behind it. If the pump under- or over-draws, the reported CFU/m³ is wrong even when the count is right, so the sampled volume must be verified against a calibrated flow meter or anemometer at defined intervals. Samplers are calibrated periodically against a traceable reference, and the sampling head — which contacts the sterile agar — must be autoclavable or disinfectable so it does not itself add contamination to the very environment it is measuring. For UK use, monitoring instruments should be maintained under a documented programme, with regulatory context from the MHRA and guidance available through GOV.UK.Specifying a microbiological air samplerMatch the achievable CFU/m³ sensitivity to your target grade — Grade A needs a full cubic-metre sample.Confirm the flow rate (e.g. 100 L/min) and that sampled volume is verifiable against a calibrated meter.Check the head design and stated d50 cut-off suit the particle sizes you must capture.Confirm an autoclavable or fully disinfectable sampling head to keep the device from adding contamination.Ensure positive-hole correction is built in or documented for the head's hole count.Verify plate compatibility (standard 90 mm and contact plates) and battery or mains options for portability.Ask for a calibration certificate and a defined re-calibration and service interval.Running costs and the verdictBeyond the instrument, the recurring cost is media — pre-poured, irradiated agar plates in the required grades — plus incubation, periodic calibration and the technician time to read and accurately record every plate. High-volume labs may automate plate set-up with automated pipetting robots to keep pace. Standardising on one sampler platform and one validated media supply keeps results comparable across sites and simplifies the validation burden. An air sampling device is worth buying well: its numbers underwrite the release of sterile products and the safety of a clean environment, so calibration, a correct capture head and reliable media matter more than headline price. To specify a microbiological air sampler and its consumables, talk to the MediGear buyers' team or get in touch.DisclaimerThis article is for informational purposes only. It is published by MediGear (medigear.uk) for general information and procurement guidance, and is not clinical, diagnostic, treatment, technical, engineering, legal or regulatory advice, nor a product endorsement, guarantee or substitute for professional assessment. MediGear does not provide medical consultations. Buyers should consult their clinical, biomedical, estates and regulatory contacts, and the manufacturer's documentation, and independently verify all specifications, certifications, compatibility and suitability before purchase. Specifications, certifications and availability are correct at the time of publication and may change without notice. MediGear is a medical-equipment distributor and does not sell medicines or pharmaceutical products.
