Welding with sparks flying
Health & Safety

Group 1: Carcinogenic to Humans

I was diagnosed with cancer a little over five years ago.

I know – that’s a bit of an opening line to a blog.

But it’s true and I am unrepentant. We are afraid of the word despite the fact that one in two people born in the UK will experience some form of cancer over their lifetime (see here).

Fortunately for me the type of cancer that I had, combined with early detection, surgery and chemotherapy have led to a very positive prognosis. I was, and am, lucky.

But it has changed my perspective on cancer. When I was a young man cancer was a disease that other people got (and frequently died from). Now I know different.

 It has also made me more sensitive to the word carcinogen.

So I was very interested to receive an e-mail notification from the HSE eBulletin service (see here) stating that enforcement expectations for mild steel welding fume have changed because all welding fume has now been reclassified as a human carcinogen. The full text of the safety alert can be read here. Note that the title of the alert just refers to mild steel but that the alert actually relates to all welding fume.

The practical upshot of the change is that all indoor welding fume must be controlled by engineering control measures. This means local exhaust ventilation (LEV – see here). Irrespective of the duration of exposure. So uncontrolled short-term exposures as a consequence of quick jobs are not acceptable. And simple dilution ventilation/ workplace ventilation will not be sufficient. If LEV can’t eliminate exposure then respiratory protective equipment (RPE – see here) will have to be used as an additional control. For outdoor work RPE will be required.


There are lots of different types of welding including arc welding (where an electric arc is struck between an electrode and the work piece to create the very high temperatures required to melt the metal) and oxyfuel gas welding where a gas torch (for example oxyacetylene) is used to provide the high temperatures required. And each of these are further divided into different specific types, such as metal inert gas (MIG) where a consumable wire electrode is melted by the electric arc within a shielding gas (such as argon) to prevent atmospheric oxygen combining with the metal to form slag.

Welding Fume

Welding fume is produced by all welding processes. Welding fume is a combination of hot gases mixed with tiny solid metal particles. The gases include oxides of nitrogen (NOx). You will have heard of these a lot over recent years because they are a major pollutant and health hazard (the VW emission scandal was about NOx). Plus carbon monoxide (CO) which is toxic on inhalation (see here). And ozone (O3 – see here).

The particulates are spheres of metal that have condensed and solidified from the vapour given off during welding. These particles are of the order of 1 micron in diameter. One micron is one thousandth of a millimetre. To put that in context the diameter of a human hair is usually stated as being 50-100 microns. So these particles are very small and will not be visible to the eye under normal lighting conditions. They can be visualised if the lighting condition are just right (see here – and look at page 9). Welding fumes may also contain other constituents that come from metal coatings (such as zinc from galvanised workpieces) and flux (chemicals added to protect the weld and enhance coalescence).

In short there are dozens of variables that interact to determine the exact composition of welding fume. And many other variables such as duration of exposure, head position relative to workpiece and ambient environmental conditions that determine what a worker’s actual exposure might be. People do PhDs in this stuff.

Human Carcinogen

By stating that welding fume is a human carcinogen the HSE safety alert is reflecting the expert view that welding fume can cause cancer.

There are lots of different types of cancer. These are often identified by reference to the organ affected, though in reality there are often multiple cancers that can affect the same organ. For example melanoma is often referred to as skin cancer. More correctly it is one type of skin cancer, named because it affects the skin pigment cells (melanocytes). The type of cancer that is definitely linked to welding fume exposure is lung cancer.

All cancers involve abnormal cell growth. Cells grow and divide inappropriately to form a tumour and this continues indefinitely. These cells can invade adjacent tissue and can spread (metastasise) in the lymphatic and circulatory systems to seed secondary tumours in more distant parts of the body. The rate at which cancer cells grow and change in nature varies. Also the ease of detecting the cancer and the effectiveness of treatments varies. Unfortunately lung cancer is one of the more common types of cancer; with low survival rates. Less than 10% of UK adults will survive for five years after diagnosis.

No Safe Level of Exposure

The thing about carcinogens is that they do not have a threshold value. For many substances there is a threshold dose below which there is no observed health effect and above which health effects can be seen. This gives rise to the classic “S- shaped” Dose/ Response curve (see here).  For carcinogens this is not the case. For carcinogens it is all about the risk of cancer. As dose increases so the risk of cancer increases. The only know safe level of exposure to a carcinogen is zero. To quote directly from the HSE Safety Alert: Regardless of duration, HSE will no longer accept any welding undertaken without any suitable exposure control measures in place, as there is no known level of safe exposure.”

Classes of Carcinogen

Carcinogens are classified into several different classes according to the available scientific evidence. The highest class is for substances where the evidence definitively shows that the substance causes cancer in humans. Lower classes are for substances where this is probable but the evidence is not so strong. Rather delightfully the classification of carcinogens is not standardised around the globe and so the EU Classification Labelling and Packaging Regs (CLP) use one system, the American Conference of Governmental Industrial Hygienist (ACGIH) uses another and the International Agency for Research on Cancer (IARC) uses a third.

The IARC Report

The IARC is the organisation responsible, in this case, for the reclassification of welding fume as a human carcinogen. IARC is a long-standing agency of the World Health Organisation (WHO). The HSE Safety Alert includes a link to the relevant IARC report (see here). The report does not present the findings of any one single toxicology or epidemiological study. Instead it is the summary of a detailed examination of hundreds of individual scientific papers. It runs to over 300 pages and is not an easy read. However certain parts are more accessible and give a flavour of the complexity of the topic. I would recommend pages 37 onwards for an overview of welding; pages 89 onwards for an overview of cancer in humans; pages 255 for the summary and page 265 for the evaluation.

Or I’ll save you the bother – the report reclassifies welding fumes as Group 1: carcinogenic to humans.

The old classification was Class 2B: possibly carcinogenic to humans. From a 1990 study (see here)

IARC Report, Courtesy of the International Agency for Research on Cancer website

UV Radiation From Welding

The study also recognises the significant risks of ultraviolet (UV) exposure to the eye and classifies UV radiation from welding as Group 1; carcinogenic to humans.

The report links welding UV radiation to ocular melanoma. This is a particular form of cancer to the eye involving the pigment cells in the iris of the eye. [I don’t recommend googling this if you are squeamish about eyes]. As a tutor I was aware of “arc-eye” – acute inflammation of the eye caused by arc flash. But I had not come across ocular melanoma before reading the IARC report.


The HSE Safety Alert also makes mention of the Workplace Health Expert Committee (WHEC). This is an advisory committee established in 2015 to give the HSE independent expert opinion on health matters. The WHEC was not involved in the IARC report but has simply recognised that the IARC report is an authoritative study whose findings need to be recognised. Further details of the WHEC can be found here.

The HSE Website

Unfortunately the HSE web pages dedicated specifically to welding (see here) are now a little out of date. They have not been updated to reflect the revised carcinogen classification nor the new HSE enforcement expectation. They also seem to completely fail to mention the UV light risk. It doesn’t appear in the health risk or safety risk lists. Instead it is mentioned incidentally in the FAQ section about welding curtains. This is an inevitable consequence of operating the labyrinthine HSE website. It’s a huge boon to anyone interested in occupation H&S. But must be a nightmare to keep up-to-date.

…And finally Ronny

Well I don’t know about you but my head hurts now. Opening up one e-mail has led to me to a dozen rabbit holes. And if you have read some of my previous blogs you will know that I can’t resist a rabbit hole.

I’ll leave you with one final thought.

The only know safe level of exposure to welding fume and UV radiation from welding is zero.

Above zero there is only pure risk. Risk of lung cancer and eye cancer. -There is no up-side.

Dr Jim Phelpstead BSc, PhD, CMIOSH

RRC Consultant Tutor