Ambient PM2.5 linked to TB reactivation in the elderly

A spatio-temporal analysis by researchers from The Chinese University of Hong Kong finds significant associations between high ambient particulate matter (PM)_2.5 concentration and increased rates of pulmonary tuberculosis (TB) among the elderly.

Hong Kong is a densely populated metropolitan city with a disproportionally high TB burden of >120 per 100,000 among people aged ≥65 years – a population where nearly all TB cases are estimated to occur due to reactivation. [Epidemiol Infect 2008;136:943-952; PLoS One 2010;5:e10468] “Facing the problems of local street-level pollution and regional smog from the Pearl River Delta region, the PM_2.5 concentration in Hong Kong generally exceeds the recommended WHO air quality guidelines and shows obvious spatio-temporal variations under the influence of monsoon climate and different types of land use across the territory,” wrote the researchers. [Air Science Group 2021: Air quality in Hong Kong, 2021] “Our aim was to profile the spatio-temporal pattern of pulmonary TB notifications among population subgroups by age in the context of its association with ambient PM_2.5 concentration.”

A total of 52,623 active pulmonary TB cases (age, ≥15 years) from a TB notification registry were included in the spatio-temporal analysis during the study period of 2005–2018. Among these, 46 percent of cases were aged ≥65 years (elderly; male-to-female ratio, 3.08:1), 23 percent were aged 50–64 years (middle-aged; male-to-female ratio, 3.17:1), and 31 percent were aged 15–49 years (adolescents and adults; male-to-female ratio, 1.07:1). Generally, pulmonary TB standardized notification ratio (SNR, ratio between the observed and expected number of cases) exhibited substantial spatio-temporal heterogeneity across Hong Kong and age groups over the years. [IJID Reg 2023;8:145-152]

For elderly pulmonary TB cases, District Council Constituency Areas (DCCAs) with SNR ≥1.5 (ie, high-risk areas with observed cases being ≥50 percent higher than expected) were mainly located in the North and Kowloon City and Eastern districts. For the middle-aged population and adolescents and adults, DCCAs with SNR ≥1.5 were mainly found in Kowloon City and Eastern districts, with high-risk pockets scattered in the North district and on Hong Kong Island.

After adjustment for socioeconomic factors, a statistically significant association between the risk of pulmonary TB and ambient PM_2.5 concentration was consistently observed among the elderly in single-pollutant models, with estimated relative risks of 1.059 in 2005–2008, 1.101 in 2009–2013, and 1.093 in 2014–2018. This suggests that upon one standard deviation increase in PM_2.5 concentration, the pulmonary TB notification rate increased by 5.9 percent, 10.1 percent, and 9.3 percent in the respective periods.

“The overall consistent, stable, and significant association patterns among the elderly suggested that increased TB reactivations might have occurred after high ambient PM_2.5 exposure. Sensitivity analysis among those aged ≥75 years revealed an even stronger pattern of association,” highlighted the researchers.

While these association patterns implied a role of ambient PM_2.5 in TB reactivations among the elderly, the unstable association patterns among the middle-aged and adolescents and adults may have been due to the larger proportion of recently infected cases or Tuberculosis Preventive Therapy in higher-risk and younger individuals.

“Another possible explanation for the discrepancy might be the differences in mobility patterns and pollutant exposure profiles,” proposed the researchers. “As compared with the elderly, the middle-aged population and adolescents and adults generally exhibited higher mobility level, spent more time in areas other than their residential locations, and had more complex exposure profiles for infections and pollutants.”