Key Takeaways
- OSHA electroplating local exhaust ventilation cyanide design starts with 1910.94(d), which is the ventilation standard for open surface tanks. Segregation of acid and cyanide sources comes before any airflow number.
- Acid plus cyanide releases hydrogen cyanide gas — the inhalation hazard is the chemistry, not the bath mist. Treat every cyanide tank as part of a segregated exhaust line until a qualified engineer says otherwise.
- The HCN operating envelope: 10 ppm PEL, 50 ppm IDLH, 4.7 ppm ceiling. These are different measurements with different meanings — do not swap them.
- Chromium tank airflow data (150–300 cfm/sq ft) does not transfer to cyanide tanks. Any cyanide-specific capture number is I-INFERENCE until it is verified against 1910.94(d) or by a qualified engineer.
- The deliverable is a written segregation and exhaust basis, not a product quote. Fill the ten-field input sheet, then have a qualified engineer review and sign it.
You are reading this because your facility runs cyanide electroplating, and someone told you the exhaust needs to be reviewed. The one-number answer you want — “this many cfm per square foot” — is exactly what this page will not give you, because the number that fits a chromium tank does not fit a cyanide tank, and 29 CFR 1910.94(d) does not hand you a single airflow figure either. The question behind “osha electroplating local exhaust ventilation cyanide” is not which scrubber to buy but how the segregation and design inputs are assembled. This page walks the cyanide-specific decision: why acid and cyanide exhaust lines must be segregated, what the official requirements actually say, and which design inputs you must collect before a qualified engineer can sign the basis.
Direct Answer: OSHA Electroplating Local Exhaust Ventilation Cyanide Segregation Comes First
Segregate the cyanide line from every acid source before you size a single hood. Hydrogen cyanide (HCN) gas forms when a cyanide solution meets an acid, and the exhaust system is the place where that mixture would be most dangerous if the two streams shared a duct. After segregation, the applicable ventilation requirement is 29 CFR 1910.94(d), “Open Surface Tanks,” which covers the tank, the hood, and the airflow — and which explicitly calls out cyanide in its special-precaution paragraph.
Choose the route this way. If your tank bath contains free or complex cyanide and any nearby bath contains acid — pickling, acid activation, acid copper — the two lines must be separate, and you start the 1910.94(d) design-input process. If the bath contains no cyanide chemistry, this page does not apply; our general electroplating ventilation design guide covers that case. If the tank is a chromium bath, stop here: the chromium airflow guidance is a different chemical system and is covered elsewhere.
| Scenario | Route | What to do next |
|---|---|---|
| Cyanide bath + acid source nearby | Segregated exhaust, 1910.94(d) inputs | Fill the ten-field input sheet, engineer review |
| Cyanide bath, no acid source on line | 1910.94(d) inputs, no segregation issue | Verify the acid-source list is truly empty, then proceed |
| No cyanide chemistry in bath | General electroplating guide | Use the boundary-page ventilation guide |
| Chromium bath | Chromium-specific guidance | Do not apply 150–300 cfm/sq ft to cyanide |
After you pick the route, you can now decide the next step: either the cyanide line is segregated and you proceed to the 1910.94(d) inputs, or it is not and the review stops until it is. The exposure limits that make segregation non-negotiable — the 10 ppm PEL and the 50 ppm IDLH — are the reason the route decision comes first.
Why Cyanide and Acid Sources Cannot Share an Exhaust System
The Chemistry: Acid Plus Cyanide Releases HCN Gas
Cyanide electroplating baths are alkaline — typically sodium cyanide or potassium cyanide solutions held at a high pH, often above 11, precisely so the cyanide stays as free or complex cyanide ion rather than as HCN gas. When an acid meets that solution, the pH drops and the equilibrium shifts: the cyanide converts to hydrogen cyanide, which boils at 25.6 °C and volatilizes out of the bath as a gas at room temperature. The inhalation hazard in cyanide plating is therefore not primarily the bath mist — it is the HCN released by accidental acid mixing, and the exposure ceiling of 4.7 ppm is what makes even a small release consequential.
This is why the exhaust design question is a chemistry question first. A shared duct that carries both an alkaline cyanide exhaust and an acid exhaust can mix the two streams inside the duct, where no operator can see the reaction and where a small acid carryover into a cyanide line can generate HCN at the fan, the scrubber, or the stack outlet. The mixing point becomes the uncontrolled release point. The first design input is therefore not a hood dimension; it is a list of every acid source on the line and a statement that no acid stream enters the cyanide exhaust.
The Regulatory Basis: No Shared Exhaust for Incompatible Streams
The segregation rule is not a recommendation. 29 CFR 1910.94(d) prohibits open-surface-tank exhaust systems from being shared when the materials in the ducts could mix to create a fire, explosion, or chemical reaction hazard — an interpretation that has been applied to cyanide and acid lines in OSHA enforcement, and the basis for the segregation requirement in electroplating facilities. The practical reading: a cyanide tank line and an acid tank line each get their own duct, their own fan, and their own discharge path, and the two do not converge anywhere upstream of the atmosphere.
The consequence of ignoring this is not a citation risk alone; it is an HCN release inside the building. Because HCN vapor has a density of 0.93 relative to air and mixes readily with it, a duct-side release can reach breathing zones far from the mixing point — and at the 50 ppm IDLH threshold, escape becomes impaired. The design input you need from this section is a one-line segregation statement — “no acid-bearing exhaust shares a duct with cyanide-bearing exhaust” — written into the exhaust basis and signed by the reviewing engineer. When you have that statement, you can decide that the isolation premise of the design is satisfied; without it, no airflow number is defensible.
The Official Requirements: OSHA Electroplating Local Exhaust Ventilation Cyanide Limits and Tank Rules
What 1910.94(d) Requires for Open Surface Tanks
29 CFR 1910.94(d) is the federal ventilation standard for open surface tanks — the category that covers electroplating and anodizing tanks where the liquid surface is exposed. For cyanide tanks, the operative paragraphs are (d)(8), which requires ventilation systems to be tested at least once every six months, and (d)(10), the “special precautions for cyanide” paragraph that adds cyanide-specific conditions on top of the general open-tank rules.
The six-month testing requirement is the part facilities most often miss. The standard does not say “test once and forget it”; the system must be tested periodically and kept in good operating condition, and the six-month interval is the floor for re-verification. Each test should confirm capture at the hood, airflow at the design point, and no cross-contamination between segregated lines. The (d)(10) cyanide paragraph is where you verify before quoting any cyanide-specific airflow number — the OSHA Technical Manual’s ventilation chapter documents the cyanide precautions, and the standard’s tank classification, not chromium guidance, is the source of a defensible cyanide capture value. For the general open-tank context, our electroplating ventilation design guide covers hood selection and system basics. Having read this section, you can now state which paragraphs apply to your cyanide tank and what the testing obligation is.
The HCN Operating Envelope: PEL 10 / IDLH 50 / Ceiling 4.7
The exposure limits are the reason the design matters, and they are three different numbers for three different purposes. Do not collapse them into one.
| Limit | Value | Meaning | Source |
|---|---|---|---|
| OSHA PEL (8-hr TWA) | 10 ppm (11 mg/m³), skin | Routine exposure ceiling over a shift | 29 CFR 1910.1000 Table Z-1 |
| NIOSH IDLH | 50 ppm | Level at which escape is impaired; emergency threshold, not a control target | NIOSH IDLH documentation |
| Short-term limit (NIOSH REL / annotated Z-1 / ACGIH TLV-C) | 4.7 ppm (5 mg/m³) | STEL / ceiling, not an 8-hr average | NIOSH REL STEL; ACGIH TLV-C |
| Lethal range reference | 110–135 ppm | Possible death within 0.5–1 hour | NIOSH IDLH documentation |
The PEL of 10 ppm is the enforceable 8-hour time-weighted average, and it carries a skin notation. The short-term limit of 4.7 ppm is a different measurement — NIOSH lists it as a 5 mg/m³ STEL and ACGIH as a ceiling — it should not be exceeded even briefly, and it is the stricter of the three for a peak-exposure scenario. The IDLH of 50 ppm is not a target; it is the concentration at which a person cannot escape without impaired breathing, which is why it governs emergency planning rather than normal operation.
The design implication is that the ventilation system must keep operator exposure below the 10 ppm TWA under normal operation and below the 4.7 ppm STEL during routine work at the tank — and the six-month test in (d)(8) is what verifies the system still does that. If a facility cannot demonstrate capture that holds the short-term limit, the exhaust basis is not complete, and the engineer review in the next section is the gate that catches it. You can now decide whether your current system’s verification record is adequate against the 10 ppm / 4.7 ppm envelope.
Design Inputs That Decide the Exhaust: Tank Chemistry, Hood Type, and Airflow
Tank Chemistry: What Your Bath Actually Contains
The exhaust basis starts from the bath, not from the fan. Record the cyanide salt — sodium cyanide (NaCN) or potassium cyanide (KCN) — the bath pH, the operating temperature, and the cyanide concentration in the working solution. A warm, low-pH cyanide bath volatilizes more HCN than a cold, high-pH bath, so temperature and pH are inputs, not background details; a typical alkaline cyanide bath runs at pH 11–13 and 30–50 °C, and these values belong in the basis.
Then list every acid source on the floor within reach of the cyanide line: acid pickle tanks, acid activation baths, acid copper or acid zinc lines, and any acid dosing or storage point. The segregation statement from the previous section needs this list to be defensible — “the nearest acid source is the acid pickle line, X meters from the cyanide line, and it exhausts to a separate duct.” If the list is incomplete, the isolation claim is incomplete, and the engineer cannot sign the basis.
Hood and Capture: Lateral, Push-Pull, and Canopy Inputs
The hood type is the second input class. Open electroplating tanks are typically captured with lateral exhaust hoods at the tank lip, push-pull systems that add an air jet across the surface, or canopy hoods above the tank — and the choice changes the airflow and the capture certainty.
| Hood type | Where it captures | Typical use | Capture certainty |
|---|---|---|---|
| Lateral (rim) exhaust | At the tank lip, one or both sides | Narrow tanks, operator works over the surface | High if tank is narrow; needs high capture velocity at the lip |
| Push-pull | Air jet pushes across surface to an exhaust slot | Wide tanks where lateral alone is weak | High across the full width; needs matched push and pull |
| Canopy | Above the tank, capturing rising vapor | Tanks with strong natural rise or low operator exposure | Lower for HCN because HCN is not strongly buoyant; relies on airflow |
For HCN, the capture mechanism matters more than for a hot acid mist. HCN gas is released at the liquid surface and mixes with the air above the bath; it does not rise on its own the way steam does. A canopy hood that relies on thermal rise can miss a cool, low-HCN-velocity release, which is why lateral or push-pull capture at the surface is the more defensible choice for cyanide tanks. The airflow figure for a cyanide tank must come from 1910.94(d)’s tank classification and hood design method or be marked I-INFERENCE and verified by the reviewing engineer — it must not be copied from the chromium guidance.
Testing and Documentation: The (d)(8) Six-Month Baseline
The third input class is the test record. Because the standard requires re-testing at least every six months, the exhaust basis should include the last test date, the measured airflow at each hood, the capture velocity at the lip, and the fan static pressure — the numbers that prove the system still meets the design point. A typical rim-hood capture velocity for an open plating tank is on the order of 100–200 fpm at the lip, but for a cyanide tank that value is I-INFERENCE until the engineer confirms it against the standard’s method.
Documentation is what turns a ventilation system into a compliance record. Keep the test date, the tester, the instrument, the measured values, and any corrective action on file alongside the segregation statement and the hood design. The test must hold operator exposure below the 10 ppm PEL and the 4.7 ppm ceiling, which is why the measured values, not the design intent, are the evidence. When the engineer reviews the basis, the six-month test log is the proof that the design assumptions still hold, and the absence of a test log is a finding that blocks sign-off. You can now assemble the three input classes — bath chemistry, hood type with capture velocity, and the test record — into the fields of the input sheet in the next section.
The Segregation and Exhaust Basis Check
The Ten-Field Input Sheet
The output of this page is a ten-field input sheet you can fill and hand to a qualified safety or process engineer. Each field maps to a decision made earlier in this page.
| # | Field | What to record | Source section |
|---|---|---|---|
| 1 | Cyanide salt and concentration | NaCN or KCN, working concentration | Tank Chemistry |
| 2 | Bath pH and temperature | Operating pH, temperature | Tank Chemistry |
| 3 | Acid source list | Every acid bath, dosing, storage point on the line | Tank Chemistry |
| 4 | Segregation statement | “No acid exhaust shares a duct with cyanide exhaust” | Segregation |
| 5 | Hood type | Lateral / push-pull / canopy | Hood and Capture |
| 6 | Capture velocity at lip | Measured or design value, fpm | Hood and Capture |
| 7 | Design airflow | cfm at the hood, with basis (1910.94(d) or I-INFERENCE) | Hood and Capture |
| 8 | Last (d)(8) test date | Date of the most recent six-month test | Testing |
| 9 | Test results | Airflow, capture velocity, static pressure, corrective actions | Testing |
| 10 | Engineer review | Reviewer name, date, sign-off | This section |
Fill all ten fields, and the exhaust basis is a document an engineer can review in one sitting. Leave any field blank, and the review stops at that gap — which is the point. You can now decide, field by field, whether your facility’s basis is complete or which gap is blocking the review.
Worked Example: A Cyanide Line Next to an Acid Pretreatment Bath
Walk a concrete case. A job shop runs an alkaline cyanide zinc line at 12 oz/gal sodium cyanide, bath pH 12.0, operating at 38 °C. Twenty feet away sits an acid pickle line using 10% sulfuric acid at 60 °C, with its own rim exhaust. The two lines currently exhaust to separate roof fans, and no duct crosses between them.
The check proceeds down the spine. Chemistry: the acid pickle is a real acid source within the line, so the segregation statement is required and currently satisfied — two separate ducts, two fans. Requirements: the cyanide line is an open surface tank, so 1910.94(d) applies, and the last test log shows a 6-month-old airflow measurement of 3,900 cfm at the cyanide rim hood. Design inputs: the hood is lateral rim exhaust, the recorded capture velocity is 150 fpm at the lip, and the airflow basis is marked I-INFERENCE because it was sized from a general plating scrubber case study, not from the 1910.94(d) tank classification. The engineer review flags field 7: the I-INFERENCE airflow needs verification against 1910.94(d) or an endorsed calculation before sign-off.
Change one condition and the outcome changes. If the acid pickle line were moved to a shared duct with the cyanide line — or if a single fan served both — the segregation statement fails, and the basis is rejected regardless of airflow. If the bath pH drifted from 12.0 to 9.0, the HCN volatilization potential rises, and the engineer would require a re-test before the next six-month interval. The worked example shows that the airflow number is rarely the deciding field; the segregation and the test record are. You can now run your own tank through the same ten-field check and see which field blocks sign-off.
When Specialist Review Is Required
A qualified safety or process engineer must review the completed input sheet before the exhaust basis is final — this is a publishing gate for this page, and it should be a working gate for your facility. The engineer verifies that the cyanide-specific airflow is grounded in 1910.94(d) or an endorsed calculation, that the segregation statement matches the physical layout, and that the six-month test record supports the design. Their sign-off is what makes the basis defensible to an inspector or an insurer.
If your facility does not have a qualified engineer on staff who works with cyanide electroplating, the basis is not ready, and the review should be contracted before any purchase or modification. The CTA on this page is not a product link — it is an offer to review your segregation and exhaust basis. Send the filled ten-field sheet, and the review starts from your inputs, not from a catalog. You can now decide the concrete next step: the basis is either signed by a qualified engineer or it is not final, and only a signed basis moves a cyanide exhaust project forward.
What This Page Will Not Do
Chromium Tank Data Does Not Transfer to Cyanide
The most common error in this topic is borrowing airflow from chromium plating guidance. The frequently cited 150–300 cfm per square foot figure is chromium-acid-bath guidance, and it is a different chemical system with a different capture objective — chromium mist and acid mist, not HCN gas. Applying that figure to a cyanide tank produces an airflow that has no basis in the cyanide chemistry and no support in 1910.94(d)’s cyanide provisions.
This page will not give you a cyanide airflow number copied from chromium data, and it will not let a vendor’s chromium case study stand in for a cyanide design. If a number can only be traced to chromium guidance, it is marked I-INFERENCE and sent to the engineer for verification — or it is not used. You can now reject any source that offers you a cyanide number lifted from the 150–300 cfm/sq ft range.
| Data source | Applies to | Does it transfer to cyanide? |
|---|---|---|
| 150–300 cfm/sq ft (chromium acid bath) | Chromium mist capture | No — different chemical system, different capture objective |
| 1910.94(d) open-surface-tank classification | All open tanks, incl. cyanide | Yes — the governing standard |
| (d)(8) six-month test interval | All open tanks | Yes — mandatory re-verification |
| Vendor plating scrubber case study (e.g., 3,900 cfm) | General plating scale reference | Only as I-INFERENCE, engineer-verified, never as a cyanide design |
| CDC MMG / Poison Control | HCN exposure response | Yes for emergency reference — this page defers to them |
No Emergency or First-Aid Instructions
This page deliberately contains no emergency response or first-aid instructions. HCN exposure response — including the use of any antidote — is a medical decision that must follow your facility’s written emergency procedures and professional medical direction. The authoritative references for cyanide exposure and treatment are the ATSDR Medical Management Guidelines for hydrogen cyanide and your poison-control center; this page does not restate dosage, antidote, or rescue guidance from any non-authoritative source.
If HCN exposure is suspected, follow the facility emergency procedure and contact emergency medical services immediately. Nothing in this page is a substitute for that step. You can now draw the boundary: for emergency or treatment guidance, go to the ATSDR Medical Management Guidelines and your poison-control center, not to a ventilation article — and for airflow design, go to 1910.94(d) and a qualified engineer, not to chromium data. The osha electroplating local exhaust ventilation cyanide basis you assemble here — segregated, standard-grounded, engineer-signed — is the deliverable that survives review.
Frequently Asked Questions
Do cyanide plating tanks require a wet scrubber? No. 1910.94(d) does not mandate a wet scrubber for cyanide tanks; it requires a ventilated open surface tank with capture that meets the standard’s provisions — the 10 ppm PEL and the 4.7 ppm ceiling are the exposure numbers the basis must hold. A wet scrubber is one possible control device after the segregation and exhaust basis is set, and the choice belongs to the reviewing engineer — not to a default product assumption.
What records prove my segregation and exhaust basis is adequate? Three records together: the segregation statement with the acid source list, the hood design and airflow basis (from 1910.94(d) or marked I-INFERENCE), and the (d)(8) six-month test log with measured airflow, capture velocity, and corrective actions. The engineer’s sign-off on those records is what makes them adequate.
How often must the exhaust system be tested? 1910.94(d)(8) requires open-surface-tank ventilation systems to be tested at least once every six months. Each test should confirm capture at the hood, airflow at the design point, and no cross-contamination between segregated lines, and the log should record the test date, the tester, the instrument, the measured values, and any corrective action. The six-month test log is the evidence that the design assumptions still hold when the engineer reviews the basis.
What is the difference between the 10 ppm PEL, the 4.7 ppm ceiling, and the 50 ppm IDLH? The 10 ppm PEL is the enforceable 8-hour time-weighted average with a skin notation; the 4.7 ppm short-term limit (NIOSH REL STEL / ACGIH ceiling) should not be exceeded even briefly; and the 50 ppm IDLH is the escape-impaired emergency threshold, not a control target. They govern different scenarios, and swapping them produces a basis that cannot be defended.
What hood type is the more defensible choice for a cyanide tank? Lateral or push-pull capture at the bath surface. HCN is released at the liquid surface and mixes with the air above the bath; it does not rise like hot steam, so a canopy hood that relies on thermal rise can miss a cool release. Capture velocities on the order of 100–200 fpm at the rim are typical design inputs and must be marked I-INFERENCE unless verified against 1910.94(d).
Can I use chromium plating airflow figures for a cyanide tank? No. The frequently cited 150–300 cfm/sq ft figure is chromium-acid-bath guidance for mist capture — a different chemical system with a different capture objective. A cyanide capture value must come from the 1910.94(d) tank classification or be marked I-INFERENCE and verified by the reviewing engineer, not lifted from chromium data.
