The poor are less likely to contract a mysterious 'brittle' form of asthma responsible for higher death-rates among the wealthy, a new study suggested.
A study of the common respiratory disease in England found while deprivation increases the mortality risk overall, the trend was reversed for people aged five to 45 years.
Scientists were so baffled by the anomaly that they suggest it could be down to a not yet detected deadlier form of the disease which costs the British taxpayer £900m annually.
Respiratory disease dubbed the ‘British’ disease has always been associated with the poor as it was linked to foul air, polluted factories, crowded living conditions that promoted infection and exposure to moulds and other allergens.
So the study funded by Asthma UK used official socio-economic (SES) data to rank asthma death victims from 2002 to 2015 on deprivation.
It found among five to 44 year olds, those in the most deprived quintile, mortality was19 per cent lower than those in the least deprived quintile.
Yet in older adults, this pattern was reversed.
Senior Research Fellow Ramyani Gupta at St George's University of London said: "We have shown that despite well-known recent declines in national asthma mortality and hospital admission rates, significant differences by SES and region remain.
"We have also highlighted an unusual and unexpected pattern of a modest but significant decrease in deaths from asthma with increasing levels of relative deprivation in children and adults less than 45 years.
"This was not consistent with the trend in national emergency admissions, where there was a trebling with increasing deprivation, nor in self-reported levels of clinically significant asthma, where there was a doubling in prevalence, in the same age group."
There was a "clear trend" of asthma mortality and deprivation in over-75s which was "attenuated, but remained" when taking into account the effects of smoking.
Ms Gupta added: "There are well-recognised occupational causes of asthma associated with lower SES jobs for example bakers, farmers, painters, welders woodworkers.
"These, as with most factors in the literature, help explain the increased asthma in lower SES groups seen in most outcomes and age-groups here and elsewhere.
"There are few examples however of risk factors of any type for higher SES groups that might account for our mortality findings in the five to 44-year-olds."
Previous research in the Global Atlas of Asthma has suggested wealthy people are at particular risk of asthma by coming into contact with harmful chemicals at work such as latex gloves in healthcare; aldehydes in dentistry; and animal waste in laboratories.
But she added: "It is not clear how much each of these might contribute" to the baffling trend for younger deaths among the affluent.
"One speculative explanation for the anomalous socio-economic trend in younger asthma deaths is that in more affluent areas there is a higher prevalence of a less predictable (more ‘brittle’) form of asthma which occasionally progresses rapidly to a fatal outcome despite apparently adequate long-term symptomatic control.
"The higher prevalence of atopy and allergies in higher SES groups may also be a factor acting together with brittle asthma or in addition to it and it is possible that some of these cases are included within deaths coded as asthma."
The research said females accounted for 68 per cent of the 14,830 recorded asthma deaths between 2002 and 2015 in England.
More than 9.000 were people aged more than 75; nearly 4,000 were aged 45-75; and the rest five to 44.
She said: "Despite falling asthma mortality over time and decades of improvements in asthma management, the UK still suffers among the highest global mortality rates in the young and significant differences by SES and region remain across England, despite national guidelines and a health service free at the point of contact.
"These differences are even wider in emergency hospital admissions where there were more than three times as many emergency admissions in the poorest among the youngest and twice as many admissions and cases of severe symptoms among the poorest in older age groups."
Six in ten of the 542,877 emergency asthma admissions over the age of five years from 2001 to 2011 were females.
Mortality rates were highest in the West Midlands for both males and females, about a third higher than the England average.
The highest emergency admission rates occurred in the North West.
Severe asthma symptoms were highest in the North East and lowest in the South East for males and East of England for females.
The paper said: "Except for mortality in ages five to 44 years, there were broadly increased event ratios with increasing deprivation for all age-groups for mortality, hospital admissions and severe symptoms.
"All showed statistically significant linear trends except for symptoms in age 75 years and over, where smaller numbers were surveyed."
In an accompanying editorial, Dr Michael Marmot of the Department of Epidemiology and Public Health at University College London, said: "Improvements in air quality, relief of crowded living conditions with ample promotion of infection and declines in smoking could all have contributed to the marked decline in mortality from respiratory conditions noted by Gupta and colleagues.
"What we are left with, though, are clear social inequalities in mortality; those lower down the social hierarchy are more likely to experience and die from respiratory disease."
The study was published in the BMJ's Thorax.