Asthma is a major cause of disability, health resource utilization and poor quality of life for those who are affected. It is the most common chronic disease among children and young adults, particularly because of its early onset (one out of four individuals in the general population develops asthma before the age of 40 years) [1], it accounts for considerable healthcare costs and loss of work productivity [2]. In 2004, Masoli et al., and the Global Initiative for Asthma (GINA) combined data from the Phase 1 International Study of Asthma and Allergies (ISAAC) study collected in 1992-1996 and the European Community Respiratory Health Survey (ECRHS) in 1988-1994 to generate global estimates of asthma burden, which suggested that asthma prevalence ranged from a low of 0.7% in Macau to 18.4% in Scotland [3–6]. This report estimated that 300 million people worldwide had asthma, and projected that this number would increase to 400 million by 2025, as countries became more urbanized [4].

The GINA report however was at best a general estimate, as the data on which it was based came from different surveys collected between (1988 and 1996) using different sampling methodologies and asthma definitions, and recruited different age groups, The World Health Survey (WHS) [7], designed and implemented by the World Health Organization (WHO) from 2002 to 2003, employed a standardized methodology to collect information from which to estimate health of populations which would allow within and between country comparisons. This data was intended to inform policy in a wide range of countries from six continents around the world. The WHS is public data available through the WHO upon request and has previously been used to estimate the burden of other chronic diseases and risk factors for these diseases [8–13]. This standardized cross-sectional survey was implemented by 70 of the 192 WHO member states, and constituted the largest multi-country survey of asthma in adults to date. We used WHS data to estimate and compare the global and country-specific burden of asthma.

Of the 181,042 individuals who completed the WHS, a total of 178,215 (98.4%) adults in 70 countries responded to the questions related to asthma diagnosis and respiratory symptoms. The median survey response rate at the household level was 93.0%, whereas the median response rate at the individual level was 99.0%. Country-specific response rates can be found on the WHS website [16]. Overall, the question-specific response rates was high, ranging from 73.3 to 97.7%. The lower response rates observed for clinical asthma was due to low response rates in the Americas (34.1%). All other regions had response rates above 70% for clinical asthma.

Global burden of asthma

The global prevalence of doctor diagnosed asthma in adults was estimated to be 4.3% (95% CI: 4.2; 4.4). The prevalence of doctor diagnosed asthma varied widely amongst the 70 participating countries, ranging from 0.2% in China to 21.0% in Australia (Table 1 and Figure 1). Using a less stringent definition, the global prevalence of clinical asthma (or treated asthma) was 4.5% (95% CI: 4.4; 4.6). The prevalence of clinical asthma also varied widely amongst the 70 participating countries, ranging from 1.0% in Vietnam to 21.5% in Australia, representing a 21-fold global variation (Table 1 and Figure 2). The five countries with the highest prevalence of clinical asthma were Australia (21.5%), Sweden (20.2%), UK (18.2%), Netherlands (15.3%), and Brazil (13.0%). Finally, using the least stringent definition, the global prevalence of wheezing was estimated to be 8.6% (95% CI: 8.5; 8.7). The prevalence of wheezing had a 15-fold variation across the world (Table 1 and Figure 2), with the highest rates observed in Australia (27.4%), the Netherlands (22.7%), the United Kingdom (22.6%), Brazil (22.6%), and Sweden (21.6%).

		Asthma Prevalence (%)2
Region1	Country	Doctor Diagnosed Asthma	Clinical Asthma	Wheezing Symptoms
Africa	Burkina Faso	2.02	2.26	5.32
	Chad	3.68	3.94	7.64
	Comoros3	7.55	7.80	12.85
	Congo3	4.65	4.79	7.93
	Cote d'Iviore3	4.22	4.59	7.70
	Ethiopia	2.00	2.00	5.53
	Ghana	3.65	3.77	4.88
	Kenya	2.86	3.12	6.22
	Malawi	4.62	4.67	7.76
	Mali	2.65	2.82	4.77
	Mauritania	6.95	7.54	11.78
	Mauritius	3.88	3.92	6.88
	Namibia	3.16	3.39	8.14
	Senegal	3.43	3.72	8.40
	South Africa5	5.92	6.09	12.40
	Swaziland5	8.74	9.69	15.37
	Zambia4	2.83	2.96	6.25
	Zimbabwe	2.28	2.52	5.48
	Regional Sub-total	3.94	4.19	7.75
Americas	Brazil	12.44	12.98	22.56
	Dominican	9.63	9.97	12.39
	Ecuador	2.03	2.13	3.83
	Guatemala4	2.26	2.42	11.95
	Mexico	2.39	2.39	3.87
	Paraguay	6.08	6.40	12.74
	Uruguay	8.60	9.10	12.02
	Regional Sub-total	4.27	4.40	7.61
Eastern Mediterranean	Morocco5	2.76	2.84	11.65
	Pakistan	3.12	3.13	5.02
	Tunisia	2.74	2.79	7.21
	United Arab Emirates	5.30	2.79	7.21
	Regional Sub-total	2.93	2.99	7.60
Europe	Austria4	7.46	7.63	9.48
	Belgium4	9.83	10.00	17.22
	Bosnia Herzegovina5	1.30	1.41	4.01
	Crotia3	4.38	4.57	8.66
	Czech Republic	4.56	4.71	6.32
	Denmark4	9.50	10.19	15.40
	Estonia	2.00	1.99	6.94
	Finland	9.39	10.24	17.19
	France	10.43	10.59	15.20
	Georgia	2.09	2.15	4.83
	Germany4	7.58	7.55	9.25
	Greece4	6.60	6.84	10.14
	Hungary	7.66	7.66	14.72
	Ireland	9.41	9.19	11.39
	Israel	7.59	8.54	14.98
	Italy4	6.05	6.26	8.98
	Kazakhstan	1.43	1.47	3.36
	Latvia	2.70	2.70	5.90
	Luxembourg	9.16	9.44	16.63
	Netherlands4	15.17	15.32	22.71
	Norway	11.05	12.32	15.05
	Portugal	7.83	7.83	8.72
	Russia3	2.50	2.57	4.98
	Slovakia	4.11	4.10	7.41
	Slovenia4	8.70	8.66	11.91
	Spain5	6.79	7.12	12.78
	Sweden	20.09	20.18	21.60
	Turkey	2.06	2.11	11.34
	UK4	17.59	18.15	22.59
	Ukraine	2.77	2.90	11.13
	Regional Sub-total	5.1	5.28	10.71
South East Asia	Bangladesh	2.91	3.23	8.63
	India3	3.16	3.30	9.63
	Myanmar	2.36	2.41	3.47
	Nepal	2.04	2.16	14.37
	Sri Lanka	2.60	2.75	6.35
	Regional Sub-total	3.24	3.39	9.71
Western Pacific	Australia3	20.96	21.51	27.39
	China3	0.19	1.42	1.73
	Laos	2.72	3.02	5.16
	Malaysia	5.21	5.51	7.55
	Philippines	7.21	7.46	11.01
	Vietnam	0.82	1.04	2.05
	Regional Sub-total	5.85	6.17	8.88
Worldwide (95%CI)		4.27 (4.17; 4.36)	4.46 (4.36; 4.55)	8.61 (8.48; 8.74)

¹ World Health Organization definition of regions

² All asthma prevalence estimates were calculated using post satisfaction weights unless otherwise indicated

³ Post stratification weights were not available from these countries, so sampling weights were used in calculating the prevalence estimates

⁴ Weights were not provided these countries

⁵ Standard errors were missing due to a sampling unit

Using standardized WHS data obtained from the WHO, we estimated that the global prevalence of clinical asthma in adults was 4.5% and varied by as much as 21-fold amongst 70 participating countries. Amongst the population living with clinical asthma, almost one in four was a current smoker, one in two reported wheezing in the past 12 months, but one in five had never received asthma treatment. The WHS is the first standardized, representative survey which included population-based data regarding respiratory symptoms and treatment permitting estimation of the global burden of asthma in adults. This study provides the most current global estimates of the burden of asthma and shows that asthma continues to be a major public health problem worldwide.

We estimated the prevalence of asthma using three definitions ranging from the most stringent - self-reported doctor diagnosis, to the most inclusive definition - self-reported wheezing. Clinical history in combination with a reversible airway obstruction as measured by a pulmonary function test is the gold standard for diagnosing asthma. However, implementing such a standard to identify individuals with asthma is impractical considering the scale of the sample of this study, and would be extremely costly and time consuming. Our questionnaire-based classifications offer a reasonable, feasible and practical alternative. While the respiratory symptoms definition may overestimate the global asthma prevalence, the clinical definition likely underestimates disease burden in resource-poor countries with inadequate access to health care facilities and treatments. Our analyses focused mainly on the clinical asthma definition, which identifies asthma burden based on diagnosis and/or treatment. This definition likely yields a lower false positive rate compared to a symptom-based definition [17].

In 2010, Sembajwe et al. used the WHS data, and reported variations in wheezing symptoms and doctor diagnosed asthma prevalence across world regions relating them to national income [13]. All subjects aged 18 to 99 years from 64 countries were included in their study. They reported a 6% prevalence of doctor diagnosed asthma and 9.2% for current wheezing, which does not agree with our findings. The major differences between their findings and ours may be attributed to differences in study population included (ours included all 70 participating countries but limited it to participants aged 18 to 45 years old). Since our prevalence estimates were lower, it suggests that their estimates may have been biased by the inclusion of subjects with COPD as asthma.

Using the WHS data to measure the global burden of asthma offers several strengths. Firstly, the same standardized questionnaires were applied to all individuals who participated. Secondly, the survey was administered using multi-staged random sampling in most of the sites making the country-specific estimates representative of the whole population. The survey in each country was also stratified by age, sex and rural/urban residence, further improving the generalizability of our findings. Nevertheless, the WHS data only included adults, and likely underestimates the global burden, since asthma is more prevalent in children. Canada and the United States are notable absences from the Survey, however participation was voluntary and these countries elected not to take part. Therefore, our estimates likely underestimate the total global burden of asthma, but the sample of countries included in the WHS is sufficient to make statistically sound global inferences

Our results highlight that asthma continues to be a major public health concern worldwide. Applying our 4.5% clinical asthma prevalence to the current world population of 7 billion translates to 315 million individuals with asthma. However, using our 8.6% self-reported prevalence of asthma symptoms, we estimated that nearly 623 million individuals are currently living with some level of asthma-related symptoms worldwide. While proper long-term management of asthma will allow individuals with asthma to achieve good levels of control enabling them to live with good quality of life, our data indicates that asthma control is not optimal in many countries. Worldwide, nearly half of the asthma population reported wheezing in the last 12 months, and only a moderate proportion had been diagnosed and/or received treatment. In addition, the high prevalence of smoking continues to be one of the major barriers in combating the global burden of asthma. While the highest overall prevalence of asthma was observed in resource-rich countries, many resource-poor nations also have a high prevalence of this disease. This is of concern because in most such countries, resources are consumed by the pressing demands of infectious diseases and the need to provide primary care for the broader population. In many countries there is little, if any provision of the essential medications that at both individual and population level can lead to very satisfactory control of asthma. Uncontrolled asthma poses an extra weight in the burden of non-communicable disease, which constitutes a major barrier for development.

The asthma statistics from the WHS presented here may be useful to health policy and decision makers, clinicians, and researchers in designing programs and plans of action to address risk factors such as smoking and improve quality of provided for asthma and reduce the burden that it presents, through provision of treatment that is adequate, accessible, and effective.