Solder Flux and Brazing Fumes

Why this matters

Plumbers sweat copper, HVAC techs braze line sets, electricians and appliance techs solder boards. Because the flame is small and the joint takes a minute, nobody thinks of it as fume work the way they think of welding. But the fume off a torch joint is a mix of flux decomposition products and metal oxides, and depending on what is in the rod and the flux it ranges from a nuisance to an asthma trigger to cadmium, which can put someone in intensive care from a single afternoon in a closet. The difference is mostly decided at the supply counter, before the torch is lit.

Where the fume comes from

A torch joint makes fume from three sources, and each has its own hazard.

• The flux. Flux chemically cleans the metal so the filler wets it. When heated it decomposes and vaporizes, and that plume, not the metal, is most of what you see and smell on a soldered joint.
• The filler metal. Solder and brazing rod are alloys. At brazing temperatures some constituents (zinc, cadmium) boil off well below the alloy's working temperature and oxidize into fine fume.
• The base metal and anything on it. Galvanized steel gives zinc oxide. Oil, refrigerant residue, paint and plastic near the joint decompose. Refrigerant decomposition has its own card (see related: Refrigerant Decomposition and Oxygen Displacement).

The welding fume card covers metal fume in general and its controls; this card is the torch-joint slice. See related: What Welding Fume Actually Contains and What It Needs.

Soldering: the flux is the hazard

Rosin flux, used in electronics soldering and in rosin-core wire solder, is made from colophony, a natural resin from pine. Heated colophony fume is a well-documented respiratory sensitizer: repeated exposure can cause occupational asthma, which once established is triggered by small amounts and generally does not go away. The UK's Health and Safety Executive has long named it among the leading causes of occupational asthma there. For a tech who solders boards occasionally the dose is small, but a bench tech doing it daily over a board, face in the plume, is the classic case.

Plumbing paste flux is usually a water-soluble formula built on zinc chloride and ammonium chloride. Heated, it gives off acid fume (hydrogen chloride) and zinc chloride fume: sharply irritating to eyes, nose and throat, and corrosive to skin in its paste form. The paste on fingers then rubbed in an eye is the most common injury.

The solder itself. Solder for potable water has been required to be lead-free under the Safe Drinking Water Act since its 1986 amendments, defined as no more than 0.2 percent lead in solder and flux. That protects the water; it also means modern plumbing solder is tin-based and its fume hazard is mostly the flux. Older electronics and non-potable work may still use tin-lead solder. At soldering temperatures lead vaporizes very little, so the lead route for a solderer is mainly ingestion from hands, which is a hand-washing and no-eating-at-the-bench rule. See related: Lead Exposure for the Worker, Not the Occupant.

Brazing: the rod and the flux both matter

Brazing runs much hotter than soldering, above 840 F by definition, and that heat is what pulls metal into the air.

Cadmium-bearing silver alloys: do not use them. Some silver brazing alloys have historically contained cadmium, which lowers the melting range and improves flow. Cadmium boils at about 1,409 F, inside the brazing range, so it comes off as cadmium oxide fume. The hazard has three features that make it unusually dangerous on a small job:

• Delayed onset. Symptoms of acute cadmium fume poisoning (cough, chest tightness, fever, aching) typically begin hours after exposure, often after the worker has gone home, and look like flu or metal fume fever.
• Severity. Unlike metal fume fever, which passes, high cadmium fume exposure can cause chemical pneumonitis and fluid in the lungs, and it has killed people. Long-term exposure damages kidneys and is a lung carcinogen.
• A very low limit. OSHA's cadmium standards, 29 CFR 1910.1027 for general industry and 29 CFR 1926.1127 for construction, set the permissible exposure limit at 5 micrograms per cubic meter as an 8-hour average, a level a few minutes of brazing in a small space can approach.

Cadmium-free silver brazing alloys are widely available and do the same work for service trades. The fix is a purchasing rule: read the label and Section 3 of the SDS for the composition, stock only alloys stated as cadmium-free, and pull any unmarked old rod out of the truck.

Phosphorus-copper alloys (the BCuP family, with or without some silver) are the standard filler for copper-to-copper HVAC joints. They are self-fluxing on copper, so no separate flux, and contain no cadmium. Their fume is mostly copper and phosphorus oxides, an irritant at service-call volumes.

Brazing to brass, bronze or steel needs a separate flux, and that flux typically contains fluorides and borates. Heated, it releases fluoride fume, a strong irritant to eyes and airways at the joint. OSHA's limit for fluorides is 2.5 milligrams per cubic meter as fluoride, in Table Z-1 of 29 CFR 1910.1000 and the construction counterpart at 29 CFR 1926.55.

Zinc. Brass contains zinc, and galvanized steel is zinc-coated. Heating either produces zinc oxide fume, the classic cause of metal fume fever: chills, fever, aches and a metallic taste starting hours after exposure and clearing within a day or two. Recovery is usual, but metal fume fever means the dose was high, and its symptoms are close enough to cadmium poisoning that a responder cannot tell them apart. Treat any fever and chest symptoms after brazing as needing medical evaluation, and say exactly what alloy and flux were used.

Ventilation for a joint in a tight space

The problem with torch work in a closet, attic or crawlspace is not the total fume but that it has nowhere to go and the tech's face is right over the joint.

• Position first. Keep your head out of the rising plume; work from the side or below the joint where the geometry allows. This costs nothing and is the largest single reduction.
• Local exhaust beats general fans. A small fan blowing across a joint scatters fume around the room. An exhaust fan or a portable fume extractor with its inlet near the joint, discharging outside the space, removes it.
• Dilution for the room. In a closet air handler or a small utility room, open the door and set a fan in the doorway exhausting out, so make-up air comes in past the tech rather than fume sitting around them.
• Respiratory protection where ventilation cannot reach. For repeated joints in a confined pocket, a half-face respirator with P100 filters under the shop's written program per 29 CFR 1910.134 handles metal fume. A disposable dust mask is not rated for fume particles that fine unless its rating says so. See related: Filter and Cartridge Selection in Plain Terms.

A space that is genuinely enclosed (a crawlspace with one small opening) is also a fire and atmosphere question; see related: How to Ventilate a Small Space You Are Working In.

Worked example: an air handler swap in a hall closet

An HVAC tech is replacing an air handler in a hallway closet about 3 feet by 4 feet with an 8-foot ceiling, roughly 96 cubic feet of air. The job needs 6 brazed joints: 4 copper-to-copper on the line set and the evaporator stubs, and 2 copper-to-brass at the service valve and a filter drier with brass fittings. Nitrogen will flow through the lines during brazing; that practice and its own hazard belong to a sibling card (see related: Nitrogen Purging and Air You Cannot Smell).

Check the rod before the truck leaves. The tech's kit holds BCuP rod for the copper joints and a silver brazing alloy for the brass joints. The silver rod's package says cadmium-free, and Section 3 of its SDS lists silver, copper, zinc and tin with no cadmium. An unlabeled bundle of old silver rod found in the van goes into a bag marked "unknown composition, do not use" rather than into the kit.

Match the controls to each joint type.

• The 4 copper-to-copper joints need no added flux. Fume is modest. Controls: head out of the plume, door open.
• The 2 brass joints need fluoride-borate flux and the alloy carries zinc, so these are the worst fume of the day: fluoride and zinc oxide, in a 96-cubic-foot closet, with the tech's face within 2 feet of the joint.

Set up ventilation for the brass joints. A box fan in the closet doorway, blowing out into the hall and toward an open exterior door at the end, pulls fume out of the closet. The tech brazes from the side of the valve, not over it. Because the 2 joints take only a few minutes and the exhaust is real, the shop's practice is no respirator for this job. Had there been 6 or more flux joints, or a closet with no door to the hall, a P100 half-face under the program would be the call.

Fire control rides along. The closet wall behind the valve gets a heat shield, and a fire extinguisher sits at the doorway. Hot work in a closet is also a fire hazard, separate from fume.

What would change the call. If the old rod had been used and later identified as cadmium-bearing, every person in the closet that day needs to know, and anyone with fever, cough or chest tightness that evening goes to emergency care saying "cadmium fume exposure from brazing," because the delayed onset is exactly when people decide to sleep it off. If the joints were on galvanized pipe, zinc fume would dominate and ventilation becomes the main control.

The failure mode. The dangerous version of this job is the tech who finds a stick of old silver rod that flows beautifully, uses it for all 6 joints in a closed closet, feels fine, drives home, and develops chest symptoms at midnight that he takes for flu. Nothing on the day warned him.

References

• OSHA 29 CFR 1910.1027 and 29 CFR 1926.1127, cadmium
• OSHA 29 CFR 1910.1000 Table Z-1 and 29 CFR 1926.55, fluorides and zinc oxide fume limits; 29 CFR 1910.134, respiratory protection
• Safe Drinking Water Act, 1986 amendments, lead-free solder and flux for potable water
• See related: What Welding Fume Actually Contains and What It Needs; Filter and Cartridge Selection in Plain Terms; How to Ventilate a Small Space You Are Working In
• See related: Refrigerant Decomposition and Oxygen Displacement; Nitrogen Purging and the Air You Cannot Smell