# What dew point does medical air need?

**What dew point or water vapour content does medical air need?**

Under HTM 02-01 (2026), the NHS England guidance, medical air should contain no more than 67 vpm of water vapour, equivalent to an atmospheric pressure dew point of about −46 °C; dental air is accepted at −20 °C, and US systems follow NFPA 99.

Water is the contaminant that medical air dryers are there to remove. In HTM 02-01 (2026), the acceptance criterion for medical and surgical air is a water vapour content of no more than 67 vpm, equivalent to an atmospheric pressure dew point of approximately −46 °C or 0.05 mg/L. The HTM says that failure to achieve the minimum dew point standard should not be accepted.

## Atmospheric dew point and pressure dew point

The −46 °C figure is an atmospheric pressure dew point. A [pressure dew point](https://library.ultracontrolo.com/glossary/pressure-dew-point/) is measured at the working pressure, so a datasheet value must be read with its reference. The ULTRAMED datasheet shows the difference for the same treated air: −40 °C pressure dew point and −56 °C atmospheric dew point.

## Dental air

HTM 02-01 accepts dental air with the same limits as medical air except for moisture: a −20 °C dew point, or up to 1020 vpm of water vapour, as long as it can be maintained.

## United States

In the United States the rules for medical gas systems are in NFPA 99, the Health Care Facilities Code, whose current edition is 2027. The dew point limit that applies should be read in the edition adopted by the authority having jurisdiction.

## How a plant holds the dew point

HTM 02-01 asks for two dryer assemblies, each with its own dew point sensor and test point after the bacteria filter, hygrometers set to −46 °C atmospheric dew point, and an automatic changeover to the standby dryer if the duty dryer fails on dryness or pressure. The ULTRAAR SC datasheet gives a working dew point of −46 °C and a maximum dew point of −70 °C, and ULTRAAR ME has two ULTRAGEST systems that monitor and control the dew point continuously.

### Is −46 °C a pressure dew point?

No. HTM 02-01 states it as an atmospheric pressure dew point, equivalent to 67 vpm of water vapour.

### Where is the dew point checked?

HTM 02-01 tests water vapour at the compressor plant to confirm dryer performance and at the terminal units to detect water contamination of the pipeline.

## Sources

- [NHS England, HTM 02-01 Medical gas pipeline systems (2026), Part A, chapters 7, 9 and 20, 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)
- [NFPA 99, Health Care Facilities Code: current edition page](https://www.nfpa.org/codes-and-standards/nfpa-99-standard-development/99)
- [ULTRAAR SC datasheet, p. 3](https://www.ultracontrolo.com/products/ultraar-sc)
- [ULTRAAR ME datasheet, p. 2](https://www.ultracontrolo.com/products/ultraar-me)
- [ULTRAMED datasheet, p. 4](https://www.ultracontrolo.com/products/ultramed)

Reviewed: 2026-10-22. Ultra Controlo engineering. https://library.ultracontrolo.com/answers/what-dew-point-does-medical-air-need/