MDHS 14/4 – General Methods for Sampling and Gravimetric Analysis of Respirable, Thoracic and Inhalable Aerosols

The Foundation of Workplace Dust Exposure Monitoring in the UK

LEVCentral Expert Commentary

If HSG258 is the UK’s principal guidance on designing and maintaining Local Exhaust Ventilation (LEV), then MDHS 14/4 is the cornerstone of measuring whether those engineering controls are actually reducing workers’ exposure to airborne dust.

Published by the Health and Safety Executive (HSE), MDHS 14/4 provides the standard methodology for collecting and gravimetrically analysing the inhalable, thoracic and respirable fractions of airborne aerosols in the workplace. It is one of the most frequently referenced occupational hygiene methods in the UK and underpins countless workplace exposure assessments carried out under the Control of Substances Hazardous to Health (COSHH) Regulations.

Unlike LEV commissioning, which demonstrates that a ventilation system is operating as designed, MDHS 14/4 determines what workers are actually breathing. It therefore forms a vital link between engineering controls and occupational health by providing objective evidence of airborne dust exposure.

For occupational hygienists and LEV professionals alike, this is an essential document.

View HSE Guide

HSE Guide

Key Learning Points

MDHS 14/4 explains:

The principles of gravimetric dust sampling.
The definition of workplace aerosols.
The difference between:
- Inhalable fraction
- Thoracic fraction
- Respirable fraction
Selection of appropriate sampling equipment.
Personal sampling techniques.
Fixed-point sampling.
Sampling durations.
Flow rate calibration.
Filter preparation and weighing.
Gravimetric analysis.
Calculation of airborne dust concentrations.
Quality assurance.
Sources of measurement uncertainty.
Interpretation of exposure data.
Selection of appropriate sampling methods for different hazardous substances.

Understanding the Three Health-Related Aerosol Fractions

One of the most important contributions made by MDHS 14/4 is its explanation of the internationally recognised health-related particle size fractions.

Fraction	Where it Deposits	Typical Examples
Inhalable	Nose, mouth and entire respiratory tract	Wood dust, flour dust, cotton dust
Thoracic	Penetrates beyond the larynx into the chest	Welding fume, many process aerosols
Respirable	Deep gas-exchange region (alveoli) of the lungs	Respirable crystalline silica, coal dust, respirable wood dust

Selecting the correct fraction is fundamental to occupational hygiene because different Workplace Exposure Limits (WELs) apply to different particle size fractions.

Source Document Information

Organisation: Health and Safety Executive (HSE)

Document: MDHS 14/4 – General Methods for Sampling and Gravimetric Analysis of Respirable, Thoracic and Inhalable Aerosols

Series: Methods for the Determination of Hazardous Substances (MDHS)

Edition: Fourth Edition (2014)

Document Type: Occupational Hygiene Sampling Method

Primary Topics: Gravimetric Dust Sampling, Occupational Hygiene, Exposure Monitoring, Respirable Dust, Inhalable Dust, Thoracic Dust, COSHH.

Audience: Occupational Hygienists, BOHS P403 Candidates, LEV Engineers, Health & Safety Professionals, Analytical Laboratories and Exposure Monitoring Consultants.

LEVCentral Perspective

MDHS 14/4 is sometimes misunderstood as simply “the dust sampling method.”

In reality, it is much more than that.

It establishes the scientific basis for measuring the fraction of airborne particles that actually enters different regions of the respiratory system. This distinction is fundamental because occupational disease depends not simply on how much dust is present, but on where those particles are deposited after they are inhaled.

For example:

Large wood dust particles may predominantly contribute to the inhalable fraction.
Welding fume often consists largely of very fine particles.
Respirable crystalline silica presents its greatest risk because particles penetrate deep into the gas-exchange region of the lungs.

MDHS 14/4 provides the framework for measuring these different fractions consistently and accurately.

From an LEVCentral perspective, the document also highlights the close relationship between LEV engineering and occupational hygiene.

A commissioning engineer may verify:

hood capture,
duct velocities,
static pressures,
fan performance.

An occupational hygienist using MDHS 14/4 then determines whether those engineering controls have successfully reduced personal exposure.

Together, these two disciplines provide a much stronger demonstration of adequate control than either could achieve independently.

Further Resources

Recommended Learning

Thought Leadership

One of the most important messages within MDHS 14/4 is that airflow is not the same as exposure.

An LEV system may satisfy every commissioning parameter, yet workers may still experience significant exposure because of poor work practices, ineffective hood positioning or changes to the process. Conversely, relatively modest engineering controls can sometimes produce excellent exposure reduction when correctly designed and used.

This distinction lies at the heart of modern occupational hygiene.

From a LEVCentral perspective, MDHS 14/4 provides the evidence that complements good LEV engineering. Commissioning demonstrates how the system performs; gravimetric sampling demonstrates how well people are protected.

Together they form one of the strongest combinations available for demonstrating effective control under COSHH and for providing employers with defensible evidence that exposure has been reduced as far as is reasonably practicable.