Cleanroom and HVAC design for pharma plants

In short

A cleanroom is a room in which airborne particle concentration, air changes, pressure, temperature and humidity are controlled to defined limits so that a drug product is not contaminated during manufacture. The HVAC (heating, ventilation and air-conditioning) system is the engineering that delivers this: filtered air at a designed air-change rate, a pressure cascade between rooms, and controlled temperature and relative humidity. In India it is legally driven by Schedule M of the Drugs and Cosmetics Rules, 1945 (revised Schedule M notified in 2023, with phased compliance for smaller manufacturers), supported by ISO 14644 for classification and WHO/PIC/S GMP guidance for exports. It is examined in pharmaceutical engineering, plant utilities and GMP papers, and asked in interviews at Sun Pharma, Cipla, Dr Reddy's, Zydus, Lupin, Biocon, Serum Institute, Aurobindo, and by HVAC and validation contractors (Fabtech, Nicomac, Blue Star, Zeco).

Explained

Why cleanrooms and what they control

• A cleanroom controls four contaminants: viable (bacteria, fungi, spores), non-viable particulate, cross-contamination (another product, especially potent or beta-lactam), and mix-up of materials. HVAC controls the first three directly.
• Sources: people (the largest - a gowned operator sheds thousands of viable-carrying particles per minute), the product/process itself (dust from granulation, compression), air brought in, water and surfaces.
• Design philosophy is layered: containment at source (closed transfer, dust extraction, isolators/RABS) first, then dilution by clean air changes, then separation by airlocks and pressure cascade. HVAC is dilution and separation, not a substitute for containment.
• Classification refers to the particulate cleanliness of air; grade (A/B/C/D) in GMP combines a classification with viable limits and operating rules for a given operation.

Classification: ISO 14644-1 and GMP grades

• ISO 14644-1:2015 classifies by maximum particles per cubic metre at specified sizes. Class N limit: Cn = 10^N × (0.1/D)^2.08, D in µm.
• GMP grades (revised Schedule M, WHO and EU GMP Annex 1 are aligned on these numbers):

• Current Annex 1 (2022) and revised Schedule M also set a ≥5 µm limit of 29 per m³ for Grade A and B at rest; macroparticle counting has measurement caveats, so limits at 5 µm are treated as a trend/alert tool more than a pass-fail absolute.
• Three occupancy states: *as-built* (no equipment, no people), *at rest* (equipment installed and running, no personnel), *in operation* (normal production with the agreed number of operators).
• Non-sterile (oral solid dosage) areas are typically built to Grade D or a controlled non-classified (CNC) standard with typically 6-20 ACH; Grade A/B are reserved for aseptic processing and Grade C for preparation of solutions to be filtered/terminally sterilised.
• Terminally sterilised products allow a lower grade at filling (commonly Grade C background with Grade A at the point of fill) than aseptically filled products (Grade A in Grade B).

HVAC system elements and airflow design

• AHU (air handling unit) train, in order: fresh-air damper and mixing box → pre-filter (typically G4/10 µm) → fine filter (F8/5 µm) → cooling coil (chilled water, 7 °C supply) → heating coil/reheater → humidifier or dehumidifier → blower → terminal HEPA filter at or near the room.
• Filter grades: pre and fine filters are classified under ISO 16890 (EN 779 withdrawn); HEPA and ULPA under ISO 29463 (formerly EN 1822). H13 ≈ 99.95% and H14 ≈ 99.995% efficiency at MPPS; HEPA is conventionally quoted as 99.97% at 0.3 µm.
• Airflow patterns: *unidirectional (laminar)* - parallel streamlines at 0.36-0.54 m/s (0.45 m/s ± 20% at the working position) used for Grade A LAF/RLAF units; *turbulent/non-unidirectional mixed flow* - supply through terminal HEPAs with low-level returns, used for Grade B, C, D.
• Air changes per hour (ACH): indicative design values - Grade D 6-20, Grade C 20-40, Grade B 40-60, Grade A defined by velocity not ACH. ACH values are design guidance, not a regulatory number: Annex 1 and revised Schedule M require that recovery and cleanliness be *demonstrated*, so a lower ACH is acceptable if qualification data support it.
• Return air: low-level returns (150-250 mm above floor, 2-3 m spacing) sweep particles downward; ceiling return short-circuits the supply and is a design error in classified rooms.
• Fresh air fraction is sized for occupancy, exhaust make-up and pressurisation - commonly 10-20% in India, higher where dust extraction removes air.
• Recirculation vs 100% fresh air (once-through): recirculation saves large amounts of energy; once-through is mandatory for penicillins/cephalosporins/beta-lactams, cytotoxics, hormones, live organisms and potent compounds, which also need dedicated AHUs, segregated facilities and often exhaust HEPA.
• Dedicated AHUs per product-group/area are standard; a single AHU serving both a beta-lactam and a general block is a cross-contamination failure.

Pressure cascade, airlocks and layout

• Typical differential pressure between adjacent rooms of different grade: 10-15 Pa (Schedule M/WHO guidance; 12.5 Pa is a common design point). Across an open door it collapses - hence interlocks and self-closers.
• Positive cascade (cleanest room highest pressure) protects the product: aseptic core > Grade B > airlock > Grade C > corridor. Negative cascade (room at lower pressure) protects the operator and plant: sampling/dispensing booths, sterility test, beta-lactam, cytotoxic and high-potency suites - these are negative to surroundings but still supplied with HEPA-filtered air, and are kept positive to an uncontrolled outside via an airlock "bubble/sink" arrangement.
• Airlock types: *cascade* (pressure steps through), *sink* (airlock lower than both rooms - contains), *bubble* (airlock higher than both - protects both), *dual/double* for potent products.
• PAL (personnel airlock, with gowning steps and a crossover bench), MAL (material airlock), dynamic pass box (HEPA-supplied, interlocked) and static pass box for material transfer; de-gowning on a separate exit route avoids the dirty-clean crossing.
• Layout rules: unidirectional flow of men, material, product and waste; no backtracking; separate entry/exit; airlocks interlocked (electromagnetic or mechanical) so both doors cannot open together; viewing panels and flush, coved, cleanable surfaces (epoxy/PU flooring, modular puff/rockwool panels, radius coving, no exposed pipework).
• Temperature and RH: typical 20-25 °C and 45-55% RH for comfort in gowned areas; RH ≤ 25-30% for hygroscopic products, effervescent tablets, capsules and some APIs - achieved with desiccant (silica gel/molecular sieve) wheel dehumidifiers because a chilled-water coil alone cannot reach such low dew points.

Qualification, monitoring and testing

• HVAC qualification follows DQ-IQ-OQ-PQ with a validation master plan; the detailed qualification/validation framework is a separate subject - here the HVAC-specific tests matter.
• ISO 14644-3 test methods: (1) airflow volume/velocity (anemometer/balometer hood), (2) HEPA filter integrity - DOP/PAO aerosol challenge scanning with a photometer (leak if >0.01% penetration), (3) particle count classification per ISO 14644-1 with a calibrated laser particle counter, (4) pressure differential, (5) recovery test - 100:1 decay typically within 15-20 minutes for non-unidirectional rooms, (6) airflow visualisation (smoke study) with glycol/dry-ice fog, filmed in operation to prove no turbulence over the exposed product, (7) containment leak, temperature and humidity uniformity, light and noise levels.
• Sampling locations for classification: the minimum number of locations is read from Table A.1 of ISO 14644-1:2015 by room area (hypergeometric sampling); NL = √A (round up) was the 1999 edition's rule and is still widely taught, but it under-counts - 40 m² needs 10 locations by the table, not 7. Single-sample volume is at least 2 litres and 1 minute per location; the earlier 95% UCL statistic was dropped in the 2015 edition.
• Environmental monitoring (EM) of viables: settle (passive) plates, active air sampling (slit-to-agar, impaction), surface swabs/contact plates (RODAC) and personnel monitoring (glove prints, gowns). Grade A/B aseptic processing needs continuous non-viable particle monitoring and viable monitoring for the whole critical operation, with alert and action limits and trending.
• Requalification frequency is typically 6 months for Grade A/B and 12 months for Grade C/D, plus after any change (filter change, AHU modification, layout change).
• Documentation: user requirement specification, room data sheets, HVAC validation protocol, as-built drawings, air balance report, trend reports - all part of the Site Master File.

Indian codes, standards and energy

• Regulatory: Drugs and Cosmetics Act, 1940 and Rules, 1945 - Schedule M (Good Manufacturing Practices and requirements of premises, plant and equipment); the revised Schedule M notified on 28 December 2023 aligns India with WHO GMP, with phased timelines for small manufacturers; the extension for manufacturers with turnover up to ₹250 crore (granted on condition that an upgradation plan was filed) ran only to 31 December 2025, so the revised Schedule M now applies across the board. Schedule M-III style requirements apply to medical devices under the Medical Devices Rules, 2017. CDSCO and State FDAs inspect; US FDA, EMA, WHO-PQ and MHRA audit export sites.
• Pharmacopoeial: Indian Pharmacopoeia 2026 (10th edition) for sterility, microbial limits and water; WHO TRS guidance on HVAC for non-sterile dosage forms is the standard teaching reference.
• Standards: ISO 14644 series (classification, monitoring, test methods, design and construction), ISO 14698 (biocontamination), ISO 16890 (air filters), ISO 29463 (HEPA/ULPA), ISO 9001:2026 for quality systems (voluntary). Older IS/BIS cleanroom and filter numbers have been revised or withdrawn - cite the ISO number unless the current IS edition is confirmed.
• Building and energy: National Building Code of India 2016 for ventilation and fire; ECSBC 2024 (successor to ECBC 2017) for energy conservation in commercial buildings; ISHRAE/ASHRAE handbooks for load calculation; the Boilers Act, 2025 with the Indian Boiler Regulations, 2026 governs steam generation for humidification and sterilisation, and the CEA (Measures relating to Safety and Electric Supply) Regulations, 2023 the electrical installation. Occupational safety and working conditions in the plant now sit under the four Labour Codes in force from 21 November 2025, which replaced the Factories Act.
• Energy: HVAC is typically 45-65% of a formulation plant's electricity bill. Savings come from VFD fans, night/idle setback to at-rest ACH with pressure maintained, higher chilled-water temperature, heat-recovery wheels on exhaust, and low-pressure-drop filters - all permitted only if requalification shows limits are still met.

Key points

• Cleanroom - a room with controlled airborne particulate, viable count, pressure, temperature and RH, designed, constructed and operated to limit contamination.
• ISO 14644-1:2015 - classifies by particles/m³; Cn = 10^N × (0.1/D)^2.08.
• Grade A - ISO 5 in operation, 0.45 m/s ± 20% unidirectional flow, <1 CFU; the critical zone where product is exposed.
• Grade B - aseptic background to Grade A; ISO 5 at rest, ISO 7 in operation.
• Grade C and D - preparation and less critical steps; D is typical for oral solids and the background for terminally sterilised processing.
• Occupancy states - as-built, at rest, in operation; a classification claim is meaningless without the state.
• HEPA - 99.97% at 0.3 µm (H13/H14 under ISO 29463), integrity proven by PAO/DOP scan, leak limit 0.01% penetration.
• ACH - 6-20 (D), 20-40 (C), 40-60 (B); indicative design figures, justified by recovery and particle data, not by rule.
• Recovery test - 100-fold particle decay in about 15-20 minutes proves the dilution capacity of a turbulent room.
• Pressure differential - 10-15 Pa between grades; positive cascade protects product, negative protects operator.
• Airlocks - cascade, bubble and sink types; PAL and MAL with interlocked doors keep flows unidirectional.
• Once-through air - mandatory for beta-lactams, cytotoxics, hormones and live organisms; dedicated AHU and facility.
• Low-level returns - needed in classified rooms; ceiling return short-circuits the supply.
• Dehumidification - desiccant wheel for RH below about 30% (hygroscopic products, capsules, effervescents).
• Sampling locations - taken from Table A.1 of ISO 14644-1:2015 by area; the old NL = √A rule is superseded.
• Smoke study - filmed airflow visualisation in operation; the key evidence that Grade A airflow is not disturbed.
• Requalification - every 6 months for A/B, 12 months for C/D, and after change.
• Schedule M (revised 2023) - the Indian legal GMP basis for premises and HVAC; ISO 14644 supplies the test methods.

Formulas and examples

Example 1 - ACH and AHU sizing for a Grade C room. Room 8 m × 5 m × 3 m high.

• Volume V = 8 × 5 × 3 = 120 m³.
• Design ACH = 30 → Q = 30 × 120 = 3,600 m³/h = 3,600/1.699 ≈ 2,120 CFM.
• Fresh air at 15% = 540 m³/h; return air = 3,060 m³/h.
• With four terminal HEPAs of 0.6 m × 1.2 m (0.72 m² each, 2.88 m² total), face velocity = (3,600/3,600)/2.88 = 1.0/2.88 ≈ 0.35 m/s - acceptable for a turbulent-flow terminal HEPA (typically ≤0.45 m/s to limit pressure drop).

Example 2 - ISO class limit from the formula. ISO Class 7 at 0.5 µm:

• Cn = 10⁷ × (0.1/0.5)^2.08 = 10⁷ × (0.2)^2.08.
• ln 0.2 = −1.6094; × 2.08 = −3.3476; e^−3.3476 = 0.03520.
• Cn = 10⁷ × 0.03520 = 352,000 particles/m³ - matching Grade C at rest and Grade B in operation.

Example 3 - Recovery time check. A Grade C room at 30 ACH; particle count after a dust spill is 100 times the at-rest level.

• Decay constant = ACH/60 = 0.5 per minute.
• t = ln(100)/0.5 = 4.605/0.5 ≈ 9.2 minutes for a 100:1 reduction (perfect mixing).
• Comfortably inside the 15-20 minute expectation; real rooms are slower than ideal mixing, so the measured value is always used.

Example 4 - Door leakage air for pressurisation. A door 2.1 m × 1.0 m with a 4 mm gap all round (undercut included).

• Gap area A ≈ perimeter × gap = (2 × 2.1 + 2 × 1.0) × 0.004 = 6.2 × 0.004 = 0.0248 m².
• At ΔP = 15 Pa: Q = 0.827 × 0.0248 × √15 = 0.827 × 0.0248 × 3.873 ≈ 0.0794 m³/s ≈ 286 m³/h.
• So about 286 m³/h must be supplied in excess of return/exhaust in that room to hold 15 Pa - about 8% of the 3,600 m³/h supply in Example 1.

Example 5 - Sensible load and TR for the same room. Supply 3,600 m³/h at 14 °C into a room held at 22 °C.

• Q = 3,600/3,600 = 1.0 m³/s; ṁ = 1.2 × 1.0 = 1.2 kg/s.
• Qs = 1.2 × 1.005 × (22 − 14) = 9.65 kW of sensible cooling ≈ 9.65/3.517 ≈ 2.74 TR.
• If the fan absorbs 3.5 kW and adds its heat to the air, roughly 3.5 kW is added to the coil load - a real reason to keep filter pressure drops low.

Example 6 - Sampling locations. Grade B room of 40 m²: Table A.1 of ISO 14644-1:2015 gives 10 locations (the superseded √40 = 6.32 → 7 rule under-counts), each sampled for at least 1 minute/2 litres (in practice 1 m³ is drawn in Grade A/B so that a single particle does not fail the room).

Where it is used

• Sterile injectable plants: Grade A RABS/isolator filling lines inside Grade B at Serum Institute, Bharat Biotech, Gland Pharma, Zydus - designed and audited against Annex 1 and revised Schedule M.
• Oral solid dosage blocks: Grade D/CNC granulation, compression and coating rooms with dust extraction, negative dispensing booths and 25-30% RH areas - Sun Pharma, Cipla, Mankind, Torrent.
• API and intermediate plants: classified sampling/dispensing booths, once-through AHUs for cytotoxics and hormones, solvent-area ventilation interfacing with flameproof design.
• Biologics, vaccines and cell therapy: BSL-2/3 containment combined with cleanroom grades, negative-pressure suites with HEPA on exhaust.
• Job roles: HVAC design engineer and turnkey contractor (Fabtech, Nicomac, Blue Star, Zeco, Voltas), validation/CQV engineer, QA officer handling environmental monitoring trends, maintenance engineer owning filter and AHU PM schedules, regulatory affairs preparing the Site Master File.
• Adjacent sectors using the same rules: medical devices under the Medical Devices Rules, 2017, hospital operation theatres and IVF labs, semiconductor and optics fabs (ISO 1-4, far cleaner than pharma).

Interview questions

• Q: What is the difference between ISO 14644 class and GMP grade? A: ISO 14644 gives only a particle count limit for a stated particle size; a GMP grade (A/B/C/D) attaches that classification to a viable-count limit, an occupancy state and the operations allowed in that room.
• Q: Why are at-rest and in-operation limits different for Grade B but identical for Grade A? A: Grade A is the critical zone where the product is exposed, so it must stay ISO 5 even with people and machines working; the Grade B background is only a support zone and is allowed to degrade to ISO 7 in operation.
• Q: What differential pressure do you design between two adjacent grades, and what happens when the door opens? A: 10-15 Pa, commonly 12.5 Pa; on opening, ΔP collapses almost to zero, which is why doors are interlocked, airlocks are used and openings are kept short.
• Q: When is a room kept at negative pressure? A: When the hazard is to the operator or the plant - dispensing and sampling booths, beta-lactams, cytotoxics, hormones, live organisms - with HEPA-filtered supply and, usually, HEPA on exhaust and a bubble/sink airlock to the outside.
• Q: How do you prove a HEPA filter is intact after installation? A: By an in-situ integrity test: challenge the upstream face with PAO/DOP aerosol and scan the downstream face and seal with a photometer; penetration above 0.01% is a leak needing repair or replacement.
• Q: What does a recovery test tell you? A: How quickly the air change rate dilutes a contamination event - typically a 100:1 decay within 15-20 minutes for a non-unidirectional room; it validates the chosen ACH.
• Q: Why is a smoke study required and when must it be done? A: Filmed airflow visualisation shows that unidirectional airflow sweeps the exposed product without turbulence or reverse flow; it must be done in operation, with equipment running and operators doing real interventions.
• Q: Can you reduce the air change rate to save energy? A: Yes, if requalification data - particle counts in operation, recovery time, pressure cascade and EM trends - still meet limits; the regulation requires demonstrated performance, so unused rooms are commonly set back to at-rest ACH while holding pressure.
• Q: Why must beta-lactam facilities use 100% fresh air? A: Penicillin and cephalosporin residues sensitise patients at trace levels, so recirculation anywhere in the plant is unacceptable; the facility, AHU, equipment and gowning are all segregated and air is exhausted after filtration.
• Q: How do you achieve 20% RH in a capsule filling room? A: With a desiccant wheel dehumidifier downstream of the cooling coil - a chilled-water coil alone cannot reach the required dew point of roughly 0 °C or below.
• Q: How many particle-count locations for a 100 m² Grade C room, and in which state? A: 16 locations from Table A.1 of ISO 14644-1:2015 (the superseded √100 = 10 rule under-counts), classified at rest for Grade C (and monitored in operation per the EM plan).
• Q: Why are returns taken at low level and not at the ceiling? A: Supply from ceiling HEPAs with low-level returns gives top-to-bottom sweep that carries particles away from the product; a ceiling return short-circuits clean air back to the AHU and leaves the breathing/working zone poorly flushed.
• Q: What is the single biggest contamination source in a cleanroom? A: People - hence gowning, airlocks, limits on operator numbers, personnel monitoring and the shift to RABS and isolators that remove the operator from the Grade A zone.

Common mistakes

• Quoting a classification without its occupancy state - "the room is ISO 7" is incomplete unless it says at rest or in operation.
• Treating ACH numbers (20/40/60) as regulatory limits; they are design guidance, and qualification data decide acceptability.
• Confusing air-change rate with air velocity - Grade A is specified by velocity (0.45 m/s ± 20%), not ACH.
• Mixing up positive and negative cascade: assuming every clean room is positive, and forgetting that a containment room is negative to its surroundings but still fed HEPA-filtered air.
• Forgetting leakage and exhaust air in the air balance, so the design cannot hold the pressure differential.
• Unit slips: m³/h with CFM, m³/s with m³/h in velocity calculations, and particles per cubic foot (old FS 209E Class 100) with particles per cubic metre.
• Citing Federal Standard 209E classes (Class 100/10,000) as current - it was withdrawn in 2001 and replaced by ISO 14644; also citing the pre-2023 Schedule M requirements as if unchanged.
• Claiming a HEPA removes bacteria "by sterilising" the air - it removes particles, including microorganism-carrying ones, but does not kill them.

Related topics

Good Manufacturing Practice (GMP) · Process validation and equipment qualification: DQ, IQ, OQ and PQ · Sterile products and parenterals · Pharmaceutical unit operations: granulation, blending and tablet compression · Refrigeration and air-conditioning (psychrometry and load calculation) · Pharmaceutical water systems and utilities.