The number of cases of asthma has increased worldwide since 1990, however fewer people are dying from the condition, a major new study has shown.

A new Global Burden of Disease study looked at the impact of the two most common respiratory diseases worldwide - asthma and chronic obstructive pulmonary disease (COPD) - between 1990 and 2015.

It found that while the number of cases of asthma increased by 12% during this period - from 318 million in 1990 to 358 million in 2015 - the number of deaths fell during the same period by just over 26% - from 550,000 to 400,000.

Asthma is the world's most common chronic respiratory disease, followed by COPD. However, while there are twice as many cases of asthma compared to COPD, the death rate from COPD is eight times higher than the death rate from asthma.

COPD is an umbrella term for a number of chronic lung disorders, including bronchitis and emphysema. It is a progressive, disabling condition caused by a narrowing of the airways.

The study found that the number of cases of COPD increased by 44% between 1990 and 2015, while the number of deaths increased by over 11% - from 2.8 million deaths in 1990 to 3.2 million in 2015.

The researchers noted that many cases of both conditions continue to be left undiagnosed, misdiagnosed or under-treated, despite the fact that both conditions can often be treated or prevented with affordable interventions.

"COPD and asthma contribute substantially to the burden of non-communicable disease. Although much of the burden is either preventable or treatable with affordable interventions, these diseases have received less attention than other prominent non-communicable diseases like cardiovascular disease, cancer, or diabetes.

"Up-to-date information on COPD and asthma is key to policy making to improve access to, and quality of, existing interventions," commented the study's lead author, Prof Theo Vos, of the University of Washington in the US.

The study noted that as a result of the increase in cases of asthma and COPD, more people are now living with related disabilities, with people in developing regions the worst affected.

Disease burden due to COPD in 2015 was highest in Papua New Guinea, India and Lesotho, while the burden for asthma was highest in Afghanistan, Central African Republic and Fiji.

The researchers pointed out that the main risk factors for COPD were smoking and air pollution, followed by household air pollution, occupational risk (such as asbestos and diesel fumes) ozone and secondhand smoke.

They said that there needs to be more public health interventions to reduce air pollution and further reduce global smoking rates.

Meanwhile, the study noted that while the causes of asthma are less clear, they include smoking and asthma-causing allergens experienced in the workplace.

The researchers called for more studies into both conditions and emphasised that both need clearer definitions.

"The varied definitions of asthma and COPD around the world mean many people are not diagnosed or are incorrectly diagnosed. For this reason, we need much clearer understanding of how the diseases develop to help us identify cases more conclusively.

"The benefits of a simpler global definition of these diseases would mean more people were diagnosed, and could access the cheap and effective treatments that can prevent these avoidable deaths," Prof Vos said.

Details of these findings are published in the journal, The Lancet Respiratory Medicine.