Dust, Fumes and Ventilation
Exposure limits, how fast air must change, capture at the source, wood and metal dust, welding fume and filters.
What is in the air
Cutting, sanding, grinding, welding, soldering and spraying put particles, fumes and vapors into the air. Dust is solid particles made by breaking material; fume is the fine solid particles that condense from a vapor (welding fume); mist is liquid droplets (spray); vapor and gas are molecules in the air (solvent). Each harms the body differently, and the harm depends on the size of the particle: the smallest go deepest.
| Particle size (µm) | Behavior | Where it lands |
|---|---|---|
| 100 | Visible, settles in seconds | Nose and throat |
| 10 | Visible in light; settles in minutes | Nose, throat, upper airways |
| 4 | Hardly visible; respirable fraction begins | Deep into the lung |
| 1 | Invisible; stays airborne for hours | Deep lung and alveoli |
| 0.3 | Hardest to filter; stays airborne | Alveoli |
- Health effects include irritation, asthma and sensitization (wood and resin dusts), cancer (hardwood dust, hexavalent chromium, silica, asbestos), lung scarring (silica, metal fume), poisoning (lead) and metal fume fever (zinc).
- The effects are delayed. Years of low exposure cause damage that shows long after.
- Dust also burns (see Fire, Fuels and Explosion).
Exposure limits
Limits are set as a time-weighted average concentration over a working day, in milligrams per cubic meter (mg/m³) or parts per million (ppm). Limits differ by country and change over time; the table shows typical figures for orientation. Use the limits and the safety data sheet that apply to you.
| Substance | Typical 8-hour limit (illustrative) | Note |
|---|---|---|
| Wood dust (hardwood, inhalable) | 1 to 5 mg/m³ | Lower for hardwood (nasal cancer); the older US limit was 15 mg/m³ |
| Nuisance dust (not otherwise regulated) | 5 mg/m³ respirable, 15 inhalable | A low-hazard dust is not a no-hazard dust |
| Respirable crystalline silica | 0.05 mg/m³ | Stone, tile, concrete, some sand |
| Welding fume (general) | 5 mg/m³ | Stainless (chromium, nickel) and manganese have much lower limits |
| Lead | 0.05 mg/m³ | Solder, old paint; also ingestion |
| Isocyanates, solvents, formaldehyde | Parts per million, very low | See the safety data sheet (see Chemicals, Gases and Welding) |
How ventilation works
General ventilation dilutes the contaminant by replacing the air in the room. In a well-mixed room with a steady source G (mg/min), an airflow Q (m³/min) and a room volume V (m³), the concentration rises exponentially to the steady value C = G ÷ Q, with a time constant V ÷ Q.
C(t) = (G ÷ Q) (1 − e−Qt ÷ V)
Worked example: sanding in a 50 m³ shop
- The sanding releases G = 100 mg/min of airborne dust. At 3 air changes per hour (Q = 2.5 m³/min) the steady concentration is 100 ÷ 2.5 = 40 mg/m³; at 10 air changes (Q = 8.3 m³/min) it is 12 mg/m³.
- To bring this source down to 1 mg/m³ by dilution alone needs Q = 100 ÷ 1 = 100 m³/min, which is 120 air changes an hour: a gale. Dilution is not a way to control a strong dust source.
- With capture at the source that removes 95 % of the dust before it enters the room, the effective source is 5 mg/min: at 3 air changes the room level is 2.0 mg/m³, and to reach 1 mg/m³ needs only 6 air changes.
- The time to reach 63 % of the steady value is V ÷ Q = 20 minutes at 3 air changes (6 minutes at 10). After 20 minutes at 3 air changes the room is at 25 mg/m³, 63 % of the steady value.
- After the work stops the concentration falls as e−ACH·t: to 10 % takes 2.3 ÷ ACH hours: 46 minutes at 3 air changes, 14 minutes at 10, and 138 minutes at 1. Wait before taking off your mask.
- Bring air in as well as taking it out: a window or louvre opposite the fan, otherwise the fan starves and flow drops.
- Exhaust outdoors away from windows and air inlets.
- Good mixing and a path: clean air should pass the worker before the source, so that the breathing zone is upstream.
Capture at the source
A hood or tool port captures contaminant before it spreads. The air velocity in front of a plain opening falls very quickly with distance: for a round opening of area A and airflow Q the capture velocity at a distance x on the axis is about v = Q ÷ (10 x² + A) (the Dalla Valle equation).
Worked example: how close must the hood be?
- A 100 mm round extraction port (A = 0.008 m²) with a flow of 0.16 m³/s (9.6 m³/min) gives a capture velocity of 1.48 m/s at 10 cm, 0.39 m/s at 20 cm and 0.18 m/s at 30 cm: moving the hood from 10 to 30 cm away cuts the velocity by a factor of 8.
- Dust thrown off slowly needs about 0.5 m/s: the port must be within 18 cm of the source. Dust thrown out fast (a grinder, a saw) needs a hood that surrounds the source.
- Put the hood where the dust goes: behind and below a saw blade, at the tangent of a grinder wheel, across the bench for a sander.
- An enclosing hood (a booth or cabinet) beats an open one because the air has to enter through the opening at a controlled speed.
| Machine | Port | Airflow needed (typical) |
|---|---|---|
| Orbital sander | extractor at the tool | 3.0 m³/min (106 CFM) |
| Table saw | 100 mm port | 9.4 m³/min (332 CFM) |
| Planer | 100 mm port | 9.4 m³/min (332 CFM) |
| Bandsaw | 100 mm port | 5.0 m³/min (177 CFM) |
| Router table | 75 mm port | 3.0 m³/min (106 CFM) |
| Soldering fume | extractor arm | 0.5 m³/min (18 CFM) |
The duct sizes and airflow of a dust collector are in Power, Lighting and Compressed Air.
Particle size and settling
A particle falls at the settling velocity of Stokes’ law, v = ρ g d² ÷ (18 μ), which depends on the square of its diameter. The respirable particles settle so slowly that air currents keep them aloft for hours.
Worked example: how long it stays in the air
- For wood dust (density 700 kg/m³) in air (viscosity 1.8 × 10−5 Pa·s): a 50 µm particle falls at 53 mm/s (38 s to fall 2 m); a 10 µm particle at 2.12 mm/s (16 minutes to fall 2 m); a 1 µm particle at 0.021 mm/s (26 hours to fall 2 m).
- The dust you can see in the light settles fast; the dust that harms the lung hangs in the air, so a shop that looks clean can still be full of fine dust.
- This is why extraction at the tool and a good filter matter more than sweeping up.
Wood, MDF and metal dust
| Dust | Hazard | Control |
|---|---|---|
| Softwood dust | Irritation, asthma | Extraction; mask for sanding |
| Hardwood dust (oak, beech, mahogany) | Sensitization; nasal cancer from long exposure | Extraction at all machines; P100 or half-mask with HEPA for sanding |
| MDF, chipboard, plywood | Fine dust with formaldehyde resin | Extraction; mask; cut outdoors when possible |
| Metal dust (grinding steel, aluminum) | Fine particles, fire (aluminum, magnesium) | Extraction; keep sparks away from dust |
| Plastic dust and fumes | Irritating; some release toxic gas when hot | Extraction; no overheating; check the plastic |
| Painted or treated material | Lead in old paint; arsenic and copper in treated timber | Test; avoid sanding; special respirator and handling |
Welding and soldering fume
Worked example: fume from MIG welding
- A welder emitting 5 g of fume per hour (typical for MIG on steel) releases 83 mg/min. To keep the average in a 50 m³ shop below 5 mg/m³ by dilution needs Q = 83 ÷ 5 = 17 m³/min, which is 20 air changes per hour; capture near the arc does the job with much less.
- A fume extraction arm with a 150 mm hood held 30 cm from the arc needs 1 652 m³/h for a capture velocity of 0.5 m/s. The arm is only as good as its position: keep your head out of the plume (the rising column of fume and hot gas).
- Galvanized steel (zinc) causes metal fume fever; stainless steel releases hexavalent chromium and nickel, which are carcinogens; painted or plated parts can release lead and cadmium. Remove coatings first, or use a respirator and strong extraction.
- Soldering: rosin flux fume is a sensitizer (asthma); use a small extractor or a fan across the bench, and wash hands after handling lead solder (see Lasers, 3D Printers, CNC and Soldering).
- Brazing and cutting with flame: fumes from the flux (fluorides) and the coating are toxic; ventilate.
- Never weld or cut in a space with solvent vapor: chlorinated solvents form phosgene in an arc (see Chemicals, Gases and Welding).
Filters, collectors and housekeeping
Worked example: filter penetration
- A HEPA filter removes 99.97 % of 0.3 µm particles: the penetration is 0.030 %. Dust of 100 mg/m³ entering it comes out at 100 × 0.0003 = 0.03 mg/m³ (30 µg/m³).
- A cloth bag on a cheap collector may stop only half of the fine dust: 50 mg/m³ out of 100 mg/m³ in. The collector then recirculates the fine dust into the room, which is worse than no collector for the particles that matter. Use a cyclone plus a HEPA-class or high-efficiency filter, or exhaust outdoors.
- Empty the bin often and outdoors, wearing a mask.
- Do not sweep dry or blow with compressed air: it throws the fine dust back into the air. Use a HEPA vacuum or damp wipe.
- Keep surfaces clean: settled dust burns and re-suspends.
- Change filters on schedule and check the hose for blockages and leaks.
- Wash dusty clothes separately; shower after dusty work.
Silica, lead and asbestos
Some dusts are dangerous at tiny levels and need special rules. Respirable crystalline silica (cutting concrete, brick, stone, tile, some engineered stone) causes silicosis and lung cancer. Lead (old paint, some solder) poisons by inhalation and ingestion. Asbestos (old insulation, tiles, cement sheet, brake linings) causes cancer decades later.
Worked example: dry or wet cutting stone
- Dry cutting of tile makes about 5 mg/m³ of respirable dust, mostly silica: 100 times a limit of 0.05 mg/m³ (illustrative figures).
- With water at the blade the dust is about 0.2 mg/m³: still 4 times the limit but a 25-fold reduction; add extraction and a respirator.
- If you suspect asbestos or lead in an old building, stop, have it tested and use a licensed contractor. Do not sand, scrape or cut it.
Troubleshooting
| Problem | Likely cause | Try |
|---|---|---|
| Dust collector does not pick up | Hose too long or small, filter clogged, leak | Shorten and straighten the hose; clean the filter; seal joints |
| Fine dust on every surface | A collector that recirculates; no source capture | Better filter or exhaust outdoors; hoods at the tool |
| Headache or nausea in the shop | Solvent vapor, carbon monoxide, poor ventilation | Stop; get fresh air; ventilate; find the source |
| Cough or sore throat after sanding | No mask, no extraction | Mask; extraction; wet the dust where possible |
| Smoke from welding stays in the room | No capture; poor airflow | Extraction arm near the arc; open door and a fan to exhaust |
| Fan noise is too high | Small fan at high speed | Larger duct; enclose and place outside (see Noise and Hearing Protection) |
Related
- Workshop Safety: the overview of this guide.
- Personal Protective Equipment: respirators and masks.
- Fire, Fuels and Explosion: dust as a fuel.
- Power, Lighting and Compressed Air (dust collector airflow) and Laser Safety, Fire and Fumes (fume extraction).
FAQ
Is a shop vacuum good enough for dust collection?
For a sander or small tool, with a good filter (HEPA class) and a close hood, yes; for a planer or table saw you need a collector that moves far more air.
How many air changes per hour do I need?
Ventilation alone cannot control a strong source: capture at the tool first, then 6 to 10 air changes an hour as a general background, more for finishing.
Do I need a respirator if I have extraction?
For sanding and fine dust, yes: wear at least an N95 or better; extraction reduces but does not remove the exposure.
Is wood dust really dangerous?
Yes: irritation and asthma for all wood, and cancer risk for hardwood dust with long exposure; keep it out of the air.
References
- ACGIH, Industrial Ventilation: A Manual of Recommended Practice for Design (capture velocities and the Dalla Valle equation).
- OSHA 29 CFR 1910.1000 (air contaminants), 1910.1053 (respirable crystalline silica) and 1910.1025 (lead); HSE (UK) EH40 (workplace exposure limits).
- Hinds, W. C., Aerosol Technology, Wiley (Stokes settling, filtration).
Last reviewed: October 2026. Limits, airflows and efficiencies are typical: use your local limits and your equipment’s data.