{"slug":"aermod-modelling-pharmaceutical-chemical-plants","title":"Air Dispersion Modelling for Pharmaceutical and Chemical Plants in India","excerpt":"A practical guide to AERMOD studies for bulk drug and synthetic organic chemical units in India, from vent registers and building downwash to assessing HCl and solvents that have no Indian ambient standard.","date":"2026-11-12","author":"DSTechnoverse","image":"/blog-images/aermod-guides/aermod-modelling-pharmaceutical-chemical-plants-sources.svg","category":"Air Dispersion Modelling","tags":["AERMOD","Pharmaceutical Industry","Chemical Industry","Bulk Drugs","Environmental Clearance","VOC Modelling","Building Downwash","Air Dispersion Modelling"],"content":"> **Quick answer:** Air dispersion modelling for a pharmaceutical or chemical plant in India means building a complete inventory of every point, area and volume source on site (boilers, thermic fluid heaters, process vents, scrubbers, solvent recovery, incinerators, DG sets and tank farms), running AERMOD with representative meteorology and building downwash, and comparing the predicted concentrations plus baseline against NAAQS 2009. Pollutants without an Indian ambient standard, such as HCl, VOCs and specific solvents, are assessed against recognised health-based guideline values, and the method and reference used are stated clearly in the report.\n\nBulk drug and synthetic organic chemical units are demanding projects for air quality modelling. A single campus can hold a coal or biomass fired boiler, two thermic fluid heaters, a dozen reactor vents routed to scrubbers, a solvent recovery block, an incinerator for process residues, standby DG sets and a tank farm with breathing losses. Many of these sources are short, close to large buildings, and emit pollutants that the Indian National Ambient Air Quality Standards do not cover at all.\n\nThis guide is for EIA consultants, EHS managers and proponents preparing Environmental Clearance (EC), Consent to Establish (CTE) or expansion studies for API, intermediates, specialty chemical and dye units.\n\n## Why Pharma and Chemical Plants Need Careful Modelling\n\nThe general workflow of an AERMOD study is the same for any industry. If you need the basics, our guide to [air dispersion modelling for EIA in India](/blog/air-dispersion-modelling-for-eia-india/) walks through the full process from Terms of Reference (ToR) to report. What makes pharmaceutical and chemical plants different is the combination of four things:\n\n1. **Many sources, many pollutants.** A thermal power plant may have one or two tall stacks and three criteria pollutants. A bulk drug unit can have 20 to 40 emission points, each with its own pollutant list.\n2. **Low release heights.** Process vents and scrubber outlets are often only a few metres above the roof of the production block, which puts them inside the wake of the building and close to the ground.\n3. **Pollutants without NAAQS.** Hydrogen chloride, chlorine, hydrogen bromide, methanol, toluene, dichloromethane, acetone and similar substances have no national ambient standard, yet appraisal committees and local communities still want to know whether they are safe.\n4. **Industrial clusters.** Many units sit in dense chemical estates where background concentrations are already high and cumulative impacts are a real issue.\n\nA study that treats a bulk drug unit like a single-stack boiler house will usually draw additional queries, or worse, under-predict the impacts that matter most to neighbours.\n\n![Schematic of a typical bulk drug plant showing boiler, thermic fluid heater, scrubber vents, solvent recovery, incinerator, DG sets and tank farm with typical release heights and pollutants](/blog-images/aermod-guides/aermod-modelling-pharmaceutical-chemical-plants-sources.svg)\n\n## Typical Emission Sources at Bulk Drug and Chemical Units\n\nThe first technical task is the emission inventory. Our [emission inventory guide for dispersion modelling](/blog/emission-inventory-for-dispersion-modelling/) covers the general method; the table below lists the sources specific to this sector and the way each is usually represented in AERMOD.\n\n| Source | Typical pollutants | Usual AERMOD representation | Notes |\n|---|---|---|---|\n| Steam boiler (coal, biomass briquettes, husk, FO, natural gas) | PM, SO2, NOx, CO | POINT | Often the largest mass emitter; stack height set by CPCB formula and SPCB conditions |\n| Thermic fluid heater | PM, SO2, NOx | POINT | Frequently shares a stack with the boiler or has its own short stack |\n| Process vents via scrubbers | HCl, Cl2, HBr, NH3, SO2, H2S, solvent vapours | POINT | Low height, small diameter, near ambient temperature, low buoyancy |\n| Solvent recovery and distillation vents | VOCs, specific solvents | POINT, sometimes VOLUME | Emissions depend on condenser temperature and recovery efficiency |\n| Incinerator for process residues or spent solvents | PM, SO2, NOx, HCl, CO, TOC, dioxins and furans | POINT | Treat with the same rigour as a waste incinerator |\n| DG sets (standby) | NOx, PM, CO | POINT | Short stacks; model at rated load with a clear operating assumption |\n| Tank farm (solvents, acids) | VOCs, acid mist | AREA or VOLUME, or POINT if vented to a common header | Breathing and working losses; use recognised tank emission methods |\n| Effluent treatment plant | VOCs, H2S, NH3, odour | AREA | Aeration tanks and equalisation tanks can be significant odour sources |\n| Fugitive emissions from pumps, valves, flanges | VOCs | VOLUME or AREA | Estimate using equipment counts and recognised emission factors; LDAR programmes reduce them |\n\nThree practical points help at the inventory stage (our [AERMOD input data checklist](/blog/aermod-input-data-checklist/) has a fuller list to send to the plant team):\n\n- **Use the vent register, not just the stack list.** The consent order may list only boiler and DG stacks; the engineering team usually holds a register of every reactor, dryer and receiver vent and where it is routed.\n- **Group by scrubber.** In most API blocks, many reactor vents are manifolded into a common scrubber. Model the scrubber outlet as the source and estimate its emission rate from the inlet load and an achievable removal efficiency, not from the sum of every reactor vent at peak.\n- **Separate continuous from batch.** Batch processes emit in pulses. AERMOD can handle this with hourly emission files or emission factor options, but for EC and CTE studies the usual and defensible approach is to model the maximum hourly rate as continuous for short-term averages, and a realistic time-averaged rate for annual averages. State which approach you used.\n\n## Pollutants to Model and Which Standards Apply\n\nThe pollutant list should follow the ToR, the consent conditions and the process chemistry. The table summarises what is typically modelled and the comparison basis.\n\n| Pollutant | Indian ambient standard (NAAQS 2009) | Typical comparison approach |\n|---|---|---|\n| PM10 | 100 µg/m³ (24-h), 60 µg/m³ (annual) | Incremental plus baseline against NAAQS |\n| PM2.5 | 60 µg/m³ (24-h), 40 µg/m³ (annual) | As above; derive PM2.5 fraction from fuel and control device |\n| SO2 | 80 µg/m³ (24-h), 50 µg/m³ (annual) | As above |\n| NO2 | 80 µg/m³ (24-h), 40 µg/m³ (annual) | Model NOx; state NOx to NO2 conversion assumption |\n| CO | 4 mg/m³ (1-h), 2 mg/m³ (8-h) | Usually small for boilers; relevant for incinerators and DG sets |\n| NH3 | 400 µg/m³ (24-h), 100 µg/m³ (annual) | From ammonia scrubbers, ammonia storage, some reactions |\n| Benzene | 5 µg/m³ (annual) | Where benzene is used as solvent or raw material |\n| HCl, Cl2, HBr, HF | No NAAQS | Health-based guideline values, see next section |\n| Methanol, toluene, acetone, dichloromethane, ethyl acetate, IPA and other solvents | No NAAQS | Health-based guideline values and odour thresholds |\n| H2S, mercaptans | No NAAQS | Odour and health-based guideline values |\n| Dioxins and furans (if an incinerator is present) | No NAAQS | Compare with guideline approaches used for waste incinerators |\n\nFor the criteria pollutants, the result is the standard Indian presentation: predicted incremental ground level concentration (GLC), plus measured baseline, gives the resultant concentration, which is compared with the NAAQS. Remember that 24-hourly and 8-hourly NAAQS values are to be met 98% of the time in a year, so the baseline you add should be chosen consciously (the maximum, or the 98th percentile of the monitoring data) and described in the report. Our explainer on [NAAQS standards in India](/blog/naaqs-standards-india-explained/) covers the averaging periods and compliance rules in more detail.\n\nFor NOx, full conversion to NO2 is the simple conservative assumption; AERMOD also offers ARM2, OLM and PVMRM, the last two needing ozone data and an in-stack NO2/NOx ratio. State which you used.\n\n## Handling Pollutants Without Indian Ambient Standards\n\nThis is the question that separates a careful pharma study from a generic one. NAAQS 2009 covers twelve pollutants. Bulk drug and chemical units routinely release substances outside that list. When there is no national standard, the study should still answer a simple question: is the predicted concentration at the most exposed location low enough to be acceptable for health and amenity?\n\n![Decision tree for assessing a pollutant without an Indian ambient standard, from checking NAAQS through selecting a health-based guideline value, matching the averaging period, and reporting the ratio](/blog-images/aermod-guides/aermod-modelling-pharmaceutical-chemical-plants-guideline-tree.svg)\n\n### Sources of Guideline Values\n\nSeveral kinds of recognised references exist. Describe them by type and cite the specific document and value you use in the report.\n\n- **World Health Organization guidelines.** Health-based values for a number of substances, widely accepted in India.\n- **Toxicological reference concentrations.** Agencies such as the US EPA and the US Agency for Toxic Substances and Disease Registry publish inhalation reference values for chronic and, in some cases, acute exposure.\n- **Ambient guideline lists used by regulators.** Several state and provincial regulators abroad, and UK environmental assessment levels, provide short-term and long-term screening values for hundreds of chemicals, designed for exactly this situation.\n- **Occupational exposure limits as a last resort.** Workplace limits divided by a large safety factor give a crude indicative value. Use this only when nothing better exists, and label it as a screening comparison.\n- **Odour thresholds.** For solvents and sulphur compounds, the odour detection threshold is often far lower than any health value. Our guide to [odour dispersion modelling in India](/blog/odour-dispersion-modelling-india/) explains how to handle odour separately, including short averaging times and percentiles.\n\n### A Practical Method\n\n1. **Confirm there is no Indian standard.** Check NAAQS 2009 and any specific condition in the ToR or consent. Some SPCBs set work-zone or ambient limits in consent orders for particular substances.\n2. **Select a guideline value from a recognised, current source.** Prefer health-based ambient guidelines over derived values. Record the document, edition and the basis of the value (chronic, acute, odour).\n3. **Match the averaging period.** A 1-hour acute value must be compared with the maximum 1-hour predicted concentration, and an annual chronic value with the annual average. Do not compare a 24-hour model result with an annual reference.\n4. **Add background where it exists.** If there is no measured background for a solvent, say so plainly.\n5. **Report the ratio.** Present the predicted concentration as a percentage of the guideline value at the maximum location and at the nearest sensitive receptors (villages, schools, hospitals).\n6. **Explain the result in plain language,** for example \"the maximum 1-hour HCl concentration is about 5% of the selected acute guideline value\".\n\nDo not mix values from different sources without saying so, and never present a foreign guideline as an Indian legal limit. It is a reference for judging acceptability.\n\n## Building Downwash on Compact Sites\n\nPharmaceutical and chemical plants in Indian industrial estates are often built on plots of a few acres, with tall production blocks, solvent storage sheds, utility buildings and a boiler house close together. Short stacks and vents near these structures are exactly where building downwash matters most. When the wind blows across a building, the cavity and wake on the lee side can pull a plume to the ground much closer to the source than an undisturbed plume would reach.\n\nAERMOD handles this with the PRIME algorithm using building dimensions from BPIP-PRIME. Our dedicated guide on [AERMOD building downwash and BPIP](/blog/aermod-building-downwash-bpip/) explains the theory and the input files. For this sector, the practical points are:\n\n- **Include every building that could influence a stack.** The rule of thumb is that a building can affect a stack if the stack lies within five times the lesser of the building height or projected width. On compact sites this usually means most buildings on the plot.\n- **Tiered roofs need tiers.** Enter a lower section and a taller reactor bay as tiers, not one box at the highest roof.\n- **Take building heights from drawings,** not satellite images. A 3 m error on a 12 m building changes the result.\n- **Watch the fence line.** With downwash, the maximum predicted concentration often occurs at or just beyond the boundary wall. Place a dense receptor ring along the fence line and in the first few hundred metres. Our guide to [receptor grid design](/blog/dispersion-modelling-receptor-grid-design/) covers spacing.\n- **Check GEP stack height.** Good engineering practice height is roughly building height plus 1.5 times the lesser of building height or width. Raising a vent well below it by a few metres is often the cheapest effective mitigation.\n\n## Stack Height and Emission Standards for the Sector\n\nIndian stack height requirements for boilers come from CPCB guidance, which relates stack height to the SO2 emission rate or the particulate emission rate, with a minimum height that applies regardless of the formula. Our explainer on [stack height calculation under CPCB guidelines](/blog/stack-height-calculation-cpcb-guidelines/) shows the formulas and a worked calculation. SPCBs may also specify heights in the consent order, and for DG sets there are separate stack height rules linked to the rated capacity and the height of the surrounding building.\n\nOn the emission standards side, several sets of rules can apply to a single pharmaceutical or chemical campus:\n\n- **Boiler standards** depend on fuel and capacity, and SPCBs commonly set particulate limits in the consent.\n- **Industry-specific standards** under the Environment (Protection) Rules exist for pharmaceutical and organic chemical manufacturing. The organic chemicals standards include process vent limits for substances such as HCl, Cl2, NH3 and HBr and requirements for fugitive VOC control, including leak detection and repair (LDAR). Check the current notified text for exact limits.\n- **Captive incinerators** follow the hazardous waste incineration standards, and **DG sets** follow CPCB engine norms and DG stack height rules.\n\nA defensible approach is to model at the regulatory limit concentration multiplied by the design flue gas flow, the worst lawful operating case.\n\n## EC Category and the Regulatory Pathway\n\nUnder the EIA Notification 2006, the relevant entry for most projects discussed here is item 5(f), which covers synthetic organic chemicals industry, including dyes and dye intermediates, bulk drugs and intermediates excluding drug formulations, synthetic rubbers, basic organic chemicals and other synthetic organic chemicals and chemical intermediates. Related entries cover pesticides and pesticide-specific intermediates, petrochemical complexes, petrochemical based processing, chemical fertilisers and chlor-alkali. Drug formulation units are generally outside item 5(f) and are handled through state consents.\n\nWhether a 5(f) project is Category A (appraised at the central level by the Expert Appraisal Committee) or Category B (appraised by the State Expert Appraisal Committee) has depended on factors such as whether it is located inside a notified industrial area or estate, and the notification has been amended several times, including changes aimed at bulk drug and intermediate projects. General conditions about distance from protected areas, critically polluted areas and interstate boundaries can also move a project into Category A. Always confirm the current categorisation from the latest consolidated notification and amendments, or with the concerned authority, before finalising the study scope.\n\nThe same model supports the ToR (which usually asks for incremental GLCs, isopleths and cumulative impacts), the EIA report, the public hearing where required, and the CTE and CTO applications, where the SPCB may set stack heights and limits from the results. Our guide to [air quality modelling for Consent to Establish](/blog/air-quality-modelling-consent-to-establish/) covers the SPCB stage in detail.\n\n## Clusters: Vapi, Ankleshwar, Patancheru and Baddi\n\nGujarat's chemical belt around Vapi and Ankleshwar, the bulk drug clusters around Hyderabad including Patancheru and Bollaram, and the pharmaceutical hub around Baddi in Himachal Pradesh hold many of these units. Several such areas have featured in CPCB's Comprehensive Environmental Pollution Index assessments as critically or severely polluted, which can bring additional conditions on new projects and expansions.\n\nFor modelling, the cluster setting changes three things:\n\n- **Background is high and variable.** Baseline monitoring in a chemical estate can show PM10 close to or above the 24-hour NAAQS on some days. When the baseline is already near the standard, even a small increment matters, and appraisal committees look closely at how the resultant concentration was derived.\n- **Cumulative impact is expected.** The ToR for projects in clusters often asks for the contribution of nearby industries to be considered, either by including their stacks in the model or by relying on baseline monitoring that already captures them. Our guide to [cumulative air quality impact assessment](/blog/cumulative-air-quality-impact-assessment-india/) explains both approaches and how to avoid double counting.\n- **Terrain may matter.** Gujarat estates are mostly on flat coastal plains, where flat terrain modelling is often reasonable. Baddi lies in a valley with hills on either side, where terrain elevations and valley wind patterns affect dispersion. In such settings, include terrain elevations through AERMAP and check that the meteorological data represent valley conditions. Our guide on [terrain and land use data for AERMOD](/blog/terrain-land-use-data-aermod/) covers the inputs.\n\nAERMOD needs a full year or more of hourly surface and upper air data, not just the one season of site monitoring; our [AERMET meteorological data guide](/blog/aermet-meteorological-data-preprocessing/) explains how to prepare and document it.\n\n## Worked Example: A Bulk Drug Expansion Project\n\nThe following example uses illustrative numbers only. It shows the logic of a study, not values from any real project.\n\n### The Project\n\nA bulk drug unit in a notified industrial estate proposes an expansion. After the expansion, the site will have:\n\n- A 6 TPH coal-fired boiler with a bag filter, stack height 30 m, diameter 0.9 m, exit temperature 140 °C.\n- A thermic fluid heater firing natural gas, stack height 18 m.\n- Two scrubbers on process vents: Scrubber A (HCl service) and Scrubber B (ammonia service), outlets at 15 m, about 3 m above the roof of a 12 m production block.\n- A solvent recovery block with a condenser vent at 14 m releasing mainly toluene and methanol.\n- Two 1,000 kVA standby DG sets with stacks at the height required by the DG set rules.\n\n### Step 1: Emission Rates\n\nFor the boiler, take an illustrative coal consumption of 1.1 tonnes per hour with 0.5% sulphur. Assuming all sulphur converts to SO2 (a conservative assumption that ignores retention in ash):\n\nSO2 = 1,100 kg/h × 0.005 × 2 = 11 kg/h, which is about 3.06 g/s.\n\nFor particulate matter, the illustrative bag filter design outlet concentration is multiplied by the design flue gas flow. For NOx, use a fuel-specific emission factor or the design value from the boiler supplier.\n\nFor Scrubber A, the inlet HCl load from the process is estimated from the mass balance of the relevant batch step. With an illustrative scrubber efficiency, the outlet is about 0.02 g/s at peak hour. The solvent recovery vent, estimated from vapour pressures at the condenser outlet temperature and the non-condensable gas flow, gives illustrative peaks of 0.05 g/s toluene and 0.03 g/s methanol. DG sets are modelled at rated load for short-term averages and stated annual hours for annual averages.\n\n### Step 2: Model Setup\n\n- AERMOD with regulatory default options.\n- One year of hourly surface data with upper air data processed through AERMET, plus the on-site season of monitoring used to check representativeness.\n- Building downwash for all buildings on the plot using BPIP-PRIME. The 12 m production block and a 9 m boiler house both influence the scrubber and boiler stacks.\n- Cartesian grid out to 10 km, with 50 m spacing near the fence line, 100 m spacing to 1 km, and wider spacing beyond. Discrete receptors at the six nearest villages and a school.\n- Flat terrain, justified because elevations within 5 km vary by only a few metres; NOx modelled with full conversion to NO2.\n\n### Step 3: Results for Criteria Pollutants\n\n| Pollutant and averaging period | Max incremental GLC (µg/m³) | Location of max | Baseline used (µg/m³) | Resultant (µg/m³) | NAAQS (µg/m³) |\n|---|---|---|---|---|---|\n| PM10, 24-h | 1.8 | 650 m NE | 82.0 | 83.8 | 100 |\n| PM2.5, 24-h | 1.1 | 650 m NE | 41.0 | 42.1 | 60 |\n| SO2, 24-h | 6.2 | 700 m NE | 18.0 | 24.2 | 80 |\n| NO2, 24-h | 4.9 | 300 m SW | 26.0 | 30.9 | 80 |\n\nAll values are illustrative. The baseline is the 98th percentile of the season's monitoring at the station nearest the maximum. The NO2 maximum lies close to the plant because the short DG set stacks dominate short-term NOx.\n\n### Step 4: Results for Pollutants Without NAAQS\n\n| Pollutant | Averaging period | Max predicted (µg/m³) | Selected guideline value | Percentage of guideline |\n|---|---|---|---|---|\n| HCl | 1-h | 9.0 | Acute health-based value from a recognised ambient guideline list | Report as % |\n| Toluene | 1-h and annual | 21.0 (1-h), 0.6 (annual) | Acute and chronic reference values, plus odour threshold | Report as % |\n| Methanol | 1-h | 12.0 | Acute reference value | Report as % |\n| NH3 | 24-h | 3.5 | NAAQS 400 µg/m³ (NH3 does have an Indian standard) | Under 1% |\n\nIn the report, the third column names the specific document and value, and the last gives the percentage. Every pollutant gets a comparison basis with a matching averaging period.\n\n### Step 5: Sensitivity and Mitigation\n\nBecause the HCl and toluene maxima occur within 150 m of the fence line, driven by downwash from the production block, the study tests raising Scrubber A and the solvent recovery vent from 15 m to 20 m. The illustrative result is a reduction of more than half in the maximum 1-hour concentrations, with the maximum moving further from the boundary.\n\n## Common Queries From Appraisal Committees\n\nThese questions recur in ToRs and appraisal minutes for this sector. Answering them in the first submission saves a round of additional details.\n\n| Likely query | What to include in the report |\n|---|---|\n| Which meteorological data were used and are they representative? | Station, period, distance from site, wind rose comparison with on-site season data |\n| Have all sources been included, including fugitive and process emissions? | Full vent register summary, source table with coordinates, heights, diameters, temperatures, velocities, emission rates |\n| How were emission rates derived? | Fuel analysis, emission factors, mass balance, scrubber design efficiency, regulatory limit approach |\n| What about VOCs and specific solvents? | Solvent list, emission estimates, guideline comparison, LDAR and solvent recovery plan |\n| Has building downwash been considered? | BPIP input summary, building plan with heights, statement of PRIME use |\n| What is the cumulative impact with other industries in the area? | Cumulative approach, list of nearby sources included or justification for baseline-only approach |\n| Where are the maximum GLCs relative to habitations? | Isopleths on a base map with villages, schools and hospitals marked; values at sensitive receptors |\n| How was the baseline added to the increment? | Statistic used (maximum or 98th percentile) and which station |\n| What mitigation is proposed if the margin is small? | Stack height increase, scrubber upgrades, fuel change, bag filter design, operating restrictions on DG sets |\n| Is there an odour risk to neighbouring residents? | Odour source list, odour assessment approach, complaint management plan |\n\nFor a structured checklist of what a good report should contain, see our guide to [air dispersion modelling report contents](/blog/air-dispersion-modelling-report-contents/), and to avoid the most frequent technical errors, read [common AERMOD mistakes](/blog/common-aermod-mistakes/).\n\n## Mitigation Options Worth Testing\n\nOnce the base case runs, the same setup can test mitigation as separate scenarios: raising short vents above the building wake, merging several small cool vents into one taller stack with more momentum, adding a second scrubber stage, improving solvent recovery with lower condenser temperatures or adsorption, switching boiler fuel from coal to natural gas, restricting DG set testing to daytime hours, and LDAR for fugitive VOCs. A table comparing maxima and sensitive receptor values for each scenario gives the proponent evidence for investment and grounds the environmental management plan in numbers.\n\n## How DSTechnoverse Helps\n\nWe carry out AERMOD studies for pharmaceutical, bulk drug and chemical projects, covering the full source inventory, building downwash, guideline-based assessment of pollutants without NAAQS, and cumulative scenarios for cluster locations. EIA consultancies can use our [AERMOD modelling service](/services/aermod-air-dispersion-modelling/) on a white-label basis. For teams that want to run their own point-source screening on flat terrain, [AERMOD Studio](/services/aermod-software/) runs the real US EPA AERMOD and AERMET in the browser, and we also offer [personalised AERMOD software](/services/aermod-software-provider-india/). To discuss a project, [talk to our team](/contact/).\n\n**Regional guides:** for local regulators, industrial areas and meteorological data in pharma and chemical clusters, see AERMOD modelling in [Hyderabad and Telangana](/services/aermod-modelling-hyderabad-telangana/), [Gujarat](/services/aermod-modelling-gujarat/) and [Mumbai, Pune and Maharashtra](/services/aermod-modelling-mumbai-pune-maharashtra/).\n\n## Frequently Asked Questions\n\n**Which EIA category applies to a bulk drug or API manufacturing project?** Bulk drugs and intermediates fall under item 5(f) of the EIA Notification 2006, covering synthetic organic chemicals. Whether a project is Category A or B has depended on location, especially whether it is inside a notified industrial estate, and on general conditions. The notification has been amended several times for this sector, so confirm the current position with the latest consolidated text or the concerned authority.\n\n**How do I assess HCl or solvent vapours when there is no Indian ambient standard?** Select a health-based guideline value from a recognised source, such as WHO guidance, toxicological reference concentrations or ambient screening levels used by regulators abroad. Match the averaging period, compare the maximum predicted concentration and values at sensitive receptors, and report the result as a percentage of the guideline. State clearly that the value is a reference, not an Indian legal limit.\n\n**Should process vents be modelled individually or as one scrubber outlet?** Model the physical release point. If several reactor vents are routed to one scrubber, the scrubber outlet is the source, with an emission rate based on the combined inlet load and the scrubber's achievable efficiency. Vents that discharge directly to air must be modelled individually. A clear vent register helps the appraisal committee see that nothing has been missed.\n\n**Is building downwash really important for a pharmaceutical plant?** Yes, in most cases. Scrubber and solvent recovery vents are usually only a few metres above the roof of production blocks, so they sit inside the building wake. Without downwash, AERMOD can under-predict concentrations near the fence line considerably. Including all nearby buildings through BPIP-PRIME is standard practice and is often specifically checked by reviewers.\n\n**Do I need to model VOCs for every solvent used on site?** Not necessarily every one in detail. Prioritise solvents by quantity used, volatility and toxicity, and model those that make up most of the emission or have the lowest guideline values. Present a summary table of all solvents with the reasoning for the ones selected. Committees mainly want to see that VOCs have been considered systematically rather than ignored.\n\n**What changes for a project in an industrial cluster like Vapi or Ankleshwar?** Background concentrations are usually higher, so the resultant concentration may sit closer to the NAAQS and needs careful derivation. Cumulative impacts from neighbouring units are often required in the ToR. Areas assessed as critically polluted under the CPCB index can also attract additional conditions, so check the current status of the location early.\n\n**Does the model need to include incinerators at chemical plants?** Yes. Captive incinerators for process residues and spent solvents are significant sources of acid gases, metals and dioxins and furans. They should be modelled with design flue gas data and emission rates at or below the hazardous waste incineration standards, and the results compared with NAAQS and suitable guideline values for the non-criteria pollutants.\n\n---\n\n**Need an AERMOD study or AERMOD software?** DSTechnoverse runs regulatory [AERMOD modelling studies](/services/aermod-air-dispersion-modelling/) for EC, CTE and expansion applications, for industries and as a white-label partner for EIA consultancies, and provides [AERMOD Studio](/services/aermod-software/), our AERMOD software, [personalised for your firm](/services/aermod-software-provider-india/). We are based in **Indore, Madhya Pradesh** and work across India. 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