The fundamental conversion rule is that 1% equals 10,000 ppm. Here is the conversion table scaling down from a completely pure atmosphere (100%) to a microscopic trace (1 ppm).
| Percentage (%) | Parts Per Million (ppm) |
| 100% | 1,000,000 ppm |
| 10% | 100,000 ppm |
| 1% | 10,000 ppm |
| 0.1% | 1,000 ppm |
| 0.01% | 100 ppm |
| 0.001% | 10 ppm |
| 0.0001% | 1 ppm |
Context for atmospheric testing: We typically use % by volume for bulk gases (like Oxygen at 20.9%) and flammability ranges (LEL/UEL), whereas we switch to ppm to measure toxic gas exposure limits (like OSHA PELs or ACGIH TLVs) because toxic hazards often exist at concentrations far below 1%.