Image caption A healthy and delicious dish, but modern herring processing may cause health problems in some workers

Fish has been a key source of protein for humans throughout recorded history, but getting it to the dinner table can be a dangerous business.

Modern techniques have addressed many of the hazards of old, like flesh wounds from the knives used for filleting. But they have also introduced new ones.

Now Swedish researchers are warning about the risks to workers in the herring industry of asthma and allergies.

Their study, published in the Annals of Occupational Hygiene, found high levels of fish protein in the air close to fish filleting machines, thrown out by the machines and by the high-pressure hoses used for cleaning.

These tiny particles can be inhaled deep into the lungs, triggering health problems in some workers.

Oily fish: good for the heart, not the lungs?

Herring is a popular dish in Scandinavian countries like Sweden, especially at Easter and Midsummer. In the north they prefer it with a mustard-based dressing, whereas in Stockholm pickled herring is popular.

It is an oily fish, high in omega-3 fatty acids, and known to be good for the heart - when you eat it.

Breathing it in is a different matter.

Dr Anna Dahlman-Hoglund is one of the study's authors and an occupational hygienist at Sahlgrenska University Hospital in Gothenburg. She told Health Check that over the past decade some of her patients who worked in the fishing industry had developed breathing problems, including asthma.

So she and her team decided to look at their working conditions, measuring the quantity of herring protein in the air at various locations inside a fish processing factory. Smaller monitors were worn by some of the workers.

She found workers had high exposure to the fish particles, and attributes this to the filleting machines and high-pressure cleaning jets casting particles into the air.

When these patients aren't at work they feel much better, so it's very related to their work situation. Dr Dahlman-Höglund, Occupational Hygienist, Sahlgrenska University Hospital, Gothenburg

Previous research has shown that inhaling these particles causes breathing problems, either because the tiny particles irritate the lungs and airways, or because of an allergic reaction to the protein.

This phase of the study just measured protein levels in the air, without looking for a correlation between these levels and workers' breathing problems. But Dr Dahlman-Hoglund says there is a clear link between the illnesses of some of her patients and where they work.

"When these patients aren't at work or are on vacation, they feel much better, so it's very related to their work situation."

From tennis elbow to herring lung

Certain jobs have specific health risks attached to them, and "herring lung" could be added to a long list including tennis elbow and writer's cramp.

Like the herring workers' condition, many other occupational diseases are respiratory. Hair salons, bakeries and metal factories are some of the other places where workers are at risk from breathing particles into their lungs.

Occupational asthma is now the most common work-related lung disease in the UK and accounts for an estimated 20% of cases of new adult asthma.

Proteins in the air, like those from fish and flour, are more likely to cause problems because they are allergenic, says Dr Joanna Szram, a Consultant in occupational lung disease at the Royal Brompton Hospital.

"Fish processing workers are a known at-risk occupation, seen in population studies and reported to registries in many countries including the UK."

Occupational health hazards Any kind of dusty environment can cause breathing problems. However, most clinicians agree that asthma has more specific causes. An estimated 90% or more of occupational asthma is down to breathing in particles known to cause allergies. People most at risk include bakers, food workers and those working with animals. Other causes include inhaling certain chemicals released in processes such as paint spraying and platinum refining.

But only a minority of people will suffer from asthma after breathing in fish proteins, Dr Szram says.

"In a factory population, occupational asthma generally affects a minority of the workforce, unless exposure levels are very high. In the UK, about a third of people have increased susceptibility to allergies, and their risk of developing asthma will be higher."

New machines, old factories

Sweden only has a small fish processing industry, but the new research has implications for the millions of fish processing workers all over the world.

If people have occupational asthma, they may need to find other work. However, there are ways to lower the risks by minimising exposure.

Part of the problem for the fish processing industry is that new machines are being used in old factories with poor ventilation, says Dr Dahlman-Hoglund.

The research team advised factories to improve their ventilation and use traditional methods to clean their floors instead of high pressure hoses.

In addition, the air monitors worn by the workers in the study allowed them to see when and where they were most at risk, and change how they worked accordingly.

Michaela Archer from Seafish, a UK fish industry body, says there are a number of things companies do to protect workers.

"Where people are prone to such issues, companies can provide a range of safeguards such as breathing apparatus, guards over machines, protective clothing and health and safety training," she explains.

Just knowing the problem exists is an important first step, says Dr Dahlman-Hoglund.

"Today when factories buy new machines they are not aware that they can produce so many particles. When we told the factory about it, it helped them to improve their working conditions."