Acid mist: sampling for sulfuric acid and hydrogen chloride

This page explains what the rules and common practice call for. Every site is different, and what a survey or a filing needs at your facility is something we work out with you after we see it.

OSHA's permissible exposure limit (PEL) and NIOSH's recommended exposure limit (REL) for sulfuric acid are the same: 1 milligram per cubic meter of air (mg/m³).

ACGIH, the professional body that publishes the threshold limit values (TLVs), sets its limit at 0.2 mg/m³, which is five times lower. The interesting part is not the gap, though. It is what the TLV is written for.

The TLV is for “strong inorganic acid mists containing sulfuric acid.” It is measured on the thoracic fraction, and it sits in ACGIH's A2 carcinogen category. The thoracic fraction is the part of the mist fine enough to get past the throat and into the lungs.

That carcinogen classification does not belong to sulfuric acid as a chemical. It belongs to the mist, which forms in operations like pickling, plating and battery charging, where acid is thrown into the air as fine droplets.

This gets missed all the time. A plant reads “sulfuric acid isn't a carcinogen” on a safety data sheet, which is a fair statement about the liquid, and concludes that the mist above its tank line is a burn and irritation problem. It isn't only that.

Exposure limits

Hydrogen chloride (HCl) is the gas that hydrochloric acid gives off, and its limits are in parts per million (ppm). The OSHA and NIOSH limits for sulfuric acid are full-shift time-weighted averages (TWA), while a ceiling limit is a level that must not be exceeded at any time.

Note what the hydrogen chloride limit is not. The OSHA PEL is a ceiling, not an 8-hour TWA, so there is no full-shift average to calculate. If a survey reports “HCl 8-hour TWA: 0.4 ppm,” no limit corresponds to that number, and it will look reassuring to anyone who doesn't notice.

All three hydrogen chloride limits are ceilings, which at least makes the comparison a fair one: 5 ppm from the two federal agencies and 2 ppm from ACGIH.

Where we find it

• Lead-acid battery formation and charging rooms. This is the classic sulfuric acid mist exposure, because batteries give off gas as they charge and the gassing generates the mist. Workers here are also exposed to lead, which is usually the more costly problem in the same room.
• Pickling and acid cleaning lines for steel, stainless and wire, using sulfuric and hydrochloric acid.
• Electroplating and anodizing. Sulfuric anodize tanks, chrome and copper plating, and the air agitation in the tanks that generates the mist.
• Copper electrowinning and electrorefining tankhouses. These have large open tank surfaces and generate mist continuously.
• Semiconductor plants. Wet benches and processes that use hydrogen chloride and sulfuric acid. The tasks to look at are maintenance and chemical change-out, not normal operation.
• Thermal breakdown of PVC. Hot-wire cutting, welding or torch work on PVC-coated material, and sealing and shrink work, all release hydrogen chloride from a plastic nobody thought of as an acid source.
• Acid delivery, pouring and drum handling in all of the above.

How we sample for it

The backup pad is the part most often left out. OSHA's current sampling instructions call for a backup pad treated with sodium carbonate behind the filter, although the written method (OSHA ID-113) itself describes a plain cellulose pad. The backup pad is there because sulfuric acid mist does not all stay on the filter. Some passes through, and the pad captures it.

If someone runs the job with a bare filter cassette because that is what's in the kit for metals, they still get a result. It is low, and nothing in the report will tell you so.

Isocyanate sampling has the same kind of problem. The wrong sampling media doesn't give you an answer that looks wrong. It gives you a clean answer from a dirty room.

→ Isocyanate air sampling

The hydrogen chloride sample is 15 minutes by design. That is 7.5 liters at 0.5 L/min, and the method is built around that sampling time. It fits a ceiling limit, not a full-shift average, so the whole survey depends on being in the right place at the right moment.

The right moment depends on the process:

• On a plating line: rack entry, tank charging, drag-out and chemical additions to the bath.
• In a semiconductor plant: wet bench maintenance and chemical change-out, not normal operation.
• In PVC work: the seconds after the wire heats.

Someone has to know your process well enough to identify those moments before the visit. If the planning call did not cover them, the survey is being aimed by luck.

→ How many air samples do you really need?

Battery rooms are two surveys, not one

If your acid mist question is about a lead-acid battery charging room, the sulfuric acid mist is real, but it is probably not your biggest problem.

Lead is. Formation and charging areas, battery assembly and the maintenance work around them fall under OSHA's lead standard, which has an action level of 30 micrograms per cubic meter of air (µg/m³). The lead standard also brings a monitoring schedule, medical surveillance and recordkeeping requirements. Exposure monitoring records must be kept for at least 40 years, or for the duration of the worker's employment plus 20 years, whichever is longer.

That standard has teeth in a way the acid mist limit does not, because the acid mist limit is a single line in OSHA's general table of air contaminant limits (Table Z-1).

Sampling for both in the same visit is not difficult. It takes different filters and different pumps on the same workers and the same shift, and it costs much less than two separate trips. The mistake we see is a plant that monitors acid mist carefully for years and never monitors lead in the same room.

→ Lead exposure air monitoring

The thoracic fraction problem

Two cautions are in order. First, both analytical methods in the table above (ID-113 and ID-174SG) are partially validated, not fully validated, which is worth saying on a page that sells careful work.

Second, the ACGIH comparison is not like for like. The sulfuric acid method collects the mist on a 37 mm filter with no size-selective device, while ACGIH's 0.2 mg/m³ is set on the thoracic fraction. Those are two different sampling conventions, and lining the two numbers up in one column is the kind of comparison we warn against on our wood dust and manganese pages.

What we can stand behind is this. Your result compared against the OSHA PEL and against the health-based ACGIH TLV are both worth having. The TLV is five times lower, on a sample defined in a different way, and the carcinogen classification is the real difference, not the arithmetic. A report should show it that way, and should not calculate a ratio that doesn't mean anything.