| Environmental corrosivity | Outdoor areas | Indoor areas | |
| C1 | very low | deserts, arctic regions, rural areas | heated, low-humidity environments, e.g. offices, schools |
| C2 | low | areas with low pollution, rural zones, small towns (SO₂ < 5 g/m³) | unheated spaces with variable temperature and humidity, e.g. warehouses, apartments, sports halls |
| C3 | medium | areas with low pollution, urban zones, coastal regions with low chloride deposition (SO₂: 5–30 g/m³) | moderate frequency of condensation and pollution from production processes, e.g. food processing plants, laundries, breweries, dairies |
| C4 | high | polluted urban, industrial and coastal areas, sea coasts with salt spray (SO₂ < 5 g/m³) | high frequency of condensation and significant industrial pollution, e.g. industrial plants, swimming pools |
| C5 | very high | heavily polluted industrial areas, coastal regions, and sea coasts with salt spray (SO₂: 90–250 g/m³) | condensation almost always present or high pollution from production processes |
| CX | extremely high | tropical zones, extreme industrial environments, coastal and marine areas (SO₂ > 250 g/m³) | unventilated warehouses in humid tropical regions with external pollution exposure |
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