Background Environmental variables contribute up to half of the variation in the rheumatoid arthritis (RA) susceptibility. We have recently reported that the age of RA onset (RAo) varies across latitudes around the world, starting younger around the Tropic of Cancer (1). Latitude gradients have been used as a surrogate for studying the influence of the environment on the risks of disease in order to generate hypotheses for further investigation.
Objectives This is an exploratory study to assess whether the age of RAo correlates with tropospheric pollutants (2), electromagnetic fields, Inequality-adjusted Human Development Index (I-HDI) and Health Expenditures as per national census of participating countries.
Methods The age of RAo was obtained from the GEO-RA group database that involves 2,481 patients from 41 countries. Information of the tropospheric pollutant PM10 (particulate matter 10μ), the I-HDI, and Health Expenditures per capita was obtained from the World Health Organization's reports. The average of each country's electromagnetic fields (nanotesla, nT) from the past 50 years was calculated using geographic coordinates per country through the magnetic field calculator of The National Centers of Environmental Information. Pearson's correlation and linear regression were used to evaluate the correlation of these environmental variables with the age of RAo by country.
Results Complete data sets were available in 35 of the 41 countries. Overall, the mean age of RAo was 44±4.8 years, the annual average of PM10 of 57.5±39.3 μg/m3, the Health Expenditure per capita of US $2,212±2,742, and the electromagnetic fields of 41,900±8,720 nT. The age of RAo was younger in countries with high PM10 levels (r= -0.61, p<0.01), high inequality (I-HDI, r=0.59, p<0.01), and low Health Expenditure per capita (r=0.61, p<0.01); the age of RAo did not correlate with nT. A significant linear regression equation was found [F(2,32)=13.61, p<0.01, R2=0.46] in the age of RAo, Health Expenses per capita (β=0.0007; CI 95% 0.00004 to 0.001) and PM10 levels (β=-0.044; CI 95% -0.085 to -0.002).
Conclusions The tropospheric pollutant PM10 and the components of the Health Expenditure per capita such as the provision of health services, family planning activities and nutrition activities are variables worth to further study through hypothesis-testing designs.
GEO-RA Group. Latitude gradient influences the age of onset of rheumatoid arthritis: a worldwide survey. Clin Rheumatol 2016. DOI: 10.1007/s10067–016–3481–9.
Essouma M, Noubiap JJ. Is air pollution a risk factor for rheumatoid arthritis? J Inflamm (Lond) 2015; 12:48. DOI: 10.1186/s12950–015–0092–1.
Disclosure of Interest None declared