---
kind: "unit"
title: "40"
title_heading: "Protection of Environment"
key: "chI/subchC/part50"
level: "part"
label: "Part 50"
heading: "National Primary and Secondary Ambient Air Quality Standards"
release: "ecfr-current"
date: "2026-08-27"
url: "https://uscodex.org/cfr/40/chI-subchC-part50"
sections_count: 21
---

# Part 50 — National Primary and Secondary Ambient Air Quality Standards

Title 40 C.F.R. — Protection of Environment › Chapter I — Environmental Protection Agency › Subchapter C — Air Programs

## Sections

- [§50.1. Definitions.](/cfr/40/50.1.md)
- [§50.2. Scope.](/cfr/40/50.2.md)
- [§50.3. Reference conditions.](/cfr/40/50.3.md)
- [§50.4. National primary ambient air quality standards for sulfur oxides (sulfur dioxide).](/cfr/40/50.4.md)
- [§50.5. National secondary ambient air quality standard for sulfur oxides (sulfur dioxide).](/cfr/40/50.5.md)
- [§50.6. National primary and secondary ambient air quality standards for PM 10 .](/cfr/40/50.6.md)
- [§50.7. National primary and secondary ambient air quality standards for PM 2.5 .](/cfr/40/50.7.md)
- [§50.8. National primary ambient air quality standards for carbon monoxide.](/cfr/40/50.8.md)
- [§50.9. National 1-hour primary and secondary ambient air quality standards for ozone.](/cfr/40/50.9.md)
- [§50.10. National 8-hour primary and secondary ambient air quality standards for ozone.](/cfr/40/50.10.md)
- [§50.11. National primary and secondary ambient air quality standards for oxides of nitrogen (with nitrogen dioxide as the indicator).](/cfr/40/50.11.md)
- [§50.12. National primary and secondary ambient air quality standards for lead.](/cfr/40/50.12.md)
- [§50.13. National primary and secondary ambient air quality standards for PM 2.5 .](/cfr/40/50.13.md)
- [§50.14. Treatment of air quality monitoring data influenced by exceptional events.](/cfr/40/50.14.md)
- [§50.15. National primary and secondary ambient air quality standards for ozone.](/cfr/40/50.15.md)
- [§50.16. National primary and secondary ambient air quality standards for lead.](/cfr/40/50.16.md)
- [§50.17. National primary ambient air quality standards for sulfur oxides (sulfur dioxide).](/cfr/40/50.17.md)
- [§50.18. National primary ambient air quality standards for PM 2.5 .](/cfr/40/50.18.md)
- [§50.19. National primary and secondary ambient air quality standards for ozone.](/cfr/40/50.19.md)
- [§50.20. National primary ambient air quality standards for PM 2.5 .](/cfr/40/50.20.md)
- [§50.21. National secondary ambient air quality standards for sulfur oxides (sulfur dioxide).](/cfr/40/50.21.md)