ATM environment alarm system

Good research. Let me put it in context for your specific environments:
WHO/EPA outdoor air quality (PM2.5):

• 0–12 µg/m³ — Good
• 12–35 µg/m³ — Moderate
• 35–55 µg/m³ — Unhealthy for sensitive groups
• 55+ µg/m³ — Unhealthy

EU occupational workplace limits (8-hour TWA):

• Wood dust (hardwood): 2 mg/m³ = 2000 µg/m³
• Wood dust (softwood): 5 mg/m³ = 5000 µg/m³
• Welding fume: 1 mg/m³ = 1000 µg/m³

My suggestion for your two environments:

• WarningAlarmWood Shop100 µg/m³500 µg/m³
• Welding Booth50 µg/m³200 µg/m³

Reasoning:

• Warning at 100/50 — "something is happening, check ventilation"
• Alarm at 500/200 — "stop work, evacuate if sustained"
• Well below legal limits — gives reaction time before reaching dangerous levels
  Welding fume is more toxic per µg than wood dust, hence tighter thresholds