# What are the purity limits for medical air?

**What are the limits for oxygen, CO2, CO, SO2, NOx, oil and water in medical air?**

In England, HTM 02-01 (2026) accepts medical air with 20.9% ± 0.5% oxygen and no more than 500 ppm CO2, 5 ppm CO, 1 ppm SO2, 2 ppm NO and NO2, 0.1 mg/m³ oil and 67 vpm water; the European Pharmacopoeia sets its own limits in monograph 1238.

HTM 02-01 treats medical gases as medicines, and the European Pharmacopoeia covers medical air in monograph 1238, Air, medicinal. The EDQM lists the monograph as in use, but its limit values are only in the subscription text. The figures below are the acceptance criteria that NHS England publishes in HTM 02-01 (2026), Part A, Table 34, which combines pharmacopoeial tests with the HTM's own tests on the pipeline. The HTM takes the British Pharmacopoeia monographs as the baseline minimum standard.

| Parameter | HTM 02-01 (2026) acceptance criterion for medical air |
|---|---|
| Oxygen | 20.9% ± 0.5% |
| Carbon dioxide (CO2) | ≤ 500 ppm (v/v) |
| Carbon monoxide (CO) | ≤ 5 ppm (v/v) |
| Sulphur dioxide (SO2) | ≤ 1 ppm (v/v) |
| Oxides of nitrogen (NO and NO2) | ≤ 2 ppm (v/v) |
| Oil | ≤ 0.1 mg/m³ |
| Water vapour | ≤ 67 vpm, about −46 °C atmospheric pressure dew point |
| Particles | Free from visible particles |
| Odour | No odour |

## What the limits protect against

Several limits point to specific failures. HTM 02-01 notes that increasing or fluctuating carbon dioxide readings in air can be an early sign of dryer failure or poor compressor maintenance, and that the position of the air intake has a considerable effect on carbon monoxide levels. The European Pharmacopoeia revision history for monograph 1238 notes that the oil test is only relevant when the air is produced with an oil-lubricated compressor, and the monograph also has a test for nitrogen monoxide and nitrogen dioxide.

## Surgical and dental air

HTM 02-01 applies the same criteria to surgical air. Dental air should match medical air in every parameter except moisture: up to 1020 vpm of water vapour, drier than a −20 °C dew point.

## How ULTRAAR air is specified

The ULTRAAR SC datasheet states that the plant's air quality is in accordance with the European Pharmacopoeia and gives these values for the plant: particles below 1 µm, oil in fluid phase exempt, CO below 5 ppm, SO2 below 1 ppm, NOx below 2 ppm and a working dew point of −46 °C. Where oil-lubricated screw compressors are used, the ULTRAAR ME air treatment units carry an air quality compliance certificate, issued by an independent organisation, against the parameters of the European Pharmacopoeia monograph.

### Does the oil limit apply to oil-free compressors?

The European Pharmacopoeia revision history for monograph 1238 says the oil test is only relevant when the air comes from an oil-lubricated compressor. HTM 02-01 still asks for an oil test at the plant test point of every newly installed compressed air plant.

### Where is medical air sampled?

HTM 02-01 tests oil and dryer performance at the compressor plant and tests the terminal units for water vapour, CO, CO2, SO2 and nitrogen oxides, so both the source and the pipeline are checked.

## Sources

- [NHS England, HTM 02-01 Medical gas pipeline systems (2026), Part A, chapter 20 and Table 34](https://www.england.nhs.uk/wp-content/uploads/2016/05/health-technical-memorandum-02-01-medical-gas-pipeline-systems-part-a-design-installation-validation-verification-aug-2026.pdf)
- [EDQM, Ph. Eur. monograph 1238 Air, medicinal: status and revision history](https://extranet.edqm.eu/4DLink1/Web_mono_hist?1238)
- [ULTRAAR SC datasheet, p. 3](https://www.ultracontrolo.com/products/ultraar-sc)
- [ULTRAAR ME datasheet, p. 2](https://www.ultracontrolo.com/products/ultraar-me)

Reviewed: 2026-10-22. Ultra Controlo engineering. https://library.ultracontrolo.com/answers/what-are-the-purity-limits-for-medical-air/