Acute lower respiratory infection (ALRI) is the leading cause of death in children younger than five years of age in Bangladesh (Black et al., 2010). ALRI causes approximately 21% of all deaths in children younger than five years in Southeast Asia (Black et al., 2010). Children under five years of age in Bangladesh suffer an estimated 0.2 – 0.5 episodes of ALRI per child per year (Brooks et al., 2010; Zaman et al., 1997).

The burden from ALRI extends beyond morbidity and mortality. Evidence from Kenya suggests that increased frequency of ALRI and diarrhea in toddlers aged 18–30 months was associated with lower Bayley Mental development scores at 30 months and lower cognitive scores at 5 years of age for girls (Neumann et al., 1991). High costs associated with hospitalizations from childhood ALRI can also have a negative impact on household nutrition and education. A study of children hospitalized with ALRI in Dhaka showed that 50% of families would have to reduce household spending on food and education to cover hospitalization-related costs (Alamgir et al., 2010).

In a recent meta-analysis, exposure to indoor biomass smoke was associated with a 1.8-fold increase in the risk of developing ALRI among young children (Dherani et al., 2008). Smoke from biomass cooking fires is an important source of respirable particulate matter (PM) for children in low income countries (Smith et al., 2000). Studies of both rural and urban households in Bangladesh suggest that even households that use cleaner fuels for cooking, such as natural gas, may experience indoor PM levels many times higher than health concentrations recommended by the World Health Organization (World Health Organization, 2006; Dasgupta et al., 2006).

Despite the consistent evidence that indoor exposure to PM can increase the risk of ALRI in children, the relationship remains poorly characterized. One reason for this is that all but a few epidemiologic studies studying this relationship have used household cooking with biomass fuel as a proxy for child exposure to PM rather than actual measurements of PM exposure (Dherani et al., 2008). Therefore, little is known about the concentrations of indoor PM children were exposed to in these studies or the effect of the level of exposure on the incidence of ALRI. Importantly, use of this proxy measurement for exposure also means that little is known about exposures to PM in households that burn cleaner fuels and the effect of this exposure on ALRI.

The objective of this study was to estimate the independent effect of indoor exposure to measured PM concentrations on the incidence of ALRI among children in a low-income urban community in Dhaka, Bangladesh, and to investigate how this effect varied with age.

Two hundred and sixty-two newborns were enrolled in the birth cohort and also agreed to participate in our study to investigate indoor exposure to particulate matter and the incidence of ALRI. Complete baseline characteristics were available for 257 (98%) children and these were included in the analysis. Thirty-six percent were born weighing <2500 grams. Households were crowded, with 1.8 square meters of floor space per person. Only 6% of households cooked with biomass exclusively, although 52% reported occasional use of biomass fuel, including wood, bamboo, jute and paper. (Table 1) Twenty-one percent (52/257) of households were not available for the endline survey on fuel use so they were categorized according to their fuel use behaviors captured at baseline. PM_2.5 concentrations exceeded 100 µg/m³ for a median of 5.3 hours per day (interquartile range 4.0 – 6.9) for all study homes.

The 257 children were observed for 491 child-years; each child contributed a mean of 1.9 years to the analysis. These 257 children experienced 969 episodes of ARI, 381 (39%) of which were ALRI. Sixty-six percent of children (169/257) experienced at least one ALRI by two years of age (Table 2). No children were referred to the hospital or died due to respiratory disease during the study. The majority of the 381 ALRI episodes we observed included crepitations (65%, 247/381) and 42% (158/381) included chest indrawing (Table 3). The incidence of ARI did not vary between age groups, however, there were differences in the incidence of ALRI by age. The ALRI incidence rate for children aged 0 – 11 months was 43% higher than for children aged 12 – 23 months (0.9 vs. 0.6, p-value <0.001) (Table 4).

In bivariate analysis, low birth weight was associated with a 37 – 40% increase in ALRI risk overall and within each age group (Table 5). Being wasted and male were associated with increased incidence of ALRI overall, but not within the individual age groups. Each additional hour per day, on average, that PM_2.5 exceeded 100 µg/m³ was significantly associated with a 7% increase in ALRI risk among children 0 – 11 months old. (Table 5)

In the multivariate model for children aged 0 – 23 months, being born low birth weight was associated with a 39% increase in risk for ALRI (IRR 1.39, 95% CI 1.07 – 1.82, P = 0.02). Male sex was also independently associated with a 23% (IRR 1.23, 95% CI 1.00 – 1.51, P = 0.05) increased risk in this age group. Indoor PM_2.5 concentrations were not associated with risk for ALRI among children aged 0 – 23 months (IRR 1.04, 95% CI 0.99 – 1.09). For children 0 – 11 months of age, each hour that PM_2.5 concentrations exceeded 100 µg/m³ was independently associated with a 7% increase in ALRI incidence (IRR 1.07, 95% CI 1.01 – 1.14, P = 0.03), and being born low birth weight was associated with a 46% increase in risk (IRR 1.46, 95% CI 1.03 – 2.06, P = 0.03). Neither PM exposure nor any other risk factors were independently associated with ALRI incidence in children aged 12 – 23 months. (Table 6) When we fitted the same models with a negative binomial distribution, the mean was not different from the variance indicating that the Poisson model was a good fit for the data.

In this study each hour that PM_2.5 concentrations in the space where the child slept exceeded 100 µg/m³ was independently associated with a 7% increase in incidence of ALRI among children younger than 12 months of age, but there was no association between PM_2.5 exposure and ALRI in children aged 12 – 23 months. Results from our study suggest that infants may suffer more harmful effects from PM_2.5 exposure than older children. Many authors have hypothesized that younger children may be at higher risk of poor health due to inhalation of particulate matter than older children (Mauderly, 2000; Naeher et al., 2007), either because of relatively faster ventilation compared to body size, less well developed immune responses, or relatively smaller airway volume, although scientific evidence supporting this increased risk is lacking (Mauderly, 2000; Schwartz, 2004). The incidence rate ratio for PM_2.5 exposure from the bivariate analysis remained essentially unchanged in the multivariate model suggesting a lack of confounding by known confounders, likely because children with high PM exposure in this community are similar to children with relatively lower PM exposure in terms of other risks for ALRI. It is important to note that we did not observe any hospitalizations or deaths in our study children. Children were able to access high quality care early in their illnesses which likely reduced the likelihood of them developing a more severe illness. This kind of early intervention and close management of childhood ALRI in this study provided good incidence data on physician diagnosed ALRI; however, our ability to comment on the association between PM exposure and more severe health outcomes is limited because of this early medical intervention.

Even though 42% of households burned exclusively clean fuels, PM_2.5 concentrations exceeded 100 µg/m³ for an average of 5.3 hours per day. Evidence from Bangladesh suggests that indoor exposures to particulate matter are determined both by the fuel used for cooking as well as ambient pollution levels, particularly for households that cook with cleaner fuels (Dasgupta et al., 2006). In our study community, the most important determinant of indoor PM_2.5 concentrations was season, and not cooking fuel type (Gurley et al., 2012). Additional research to better understand the contribution of ambient PM to indoor concentrations in these homes could be useful in designing interventions to reduce PM exposure and reduce ALRI in this and similar settings.

Low birth weight was common among children in this cohort (36%) and its effect on ALRI risk was substantial. There are numerous etiologies of low birth weight (Valero De Bernabe et al., 2004) and some study findings suggest that exposure to indoor air pollution could be one contributor (Shah and Balkhair, 2011; Stieb et al., 2012). The relationship between indoor exposure to PM and risk of low birth weight should be explored in this setting. Nutritional interventions for pregnant mothers have been shown to be effective at preventing low birth weight and these should be implemented to prevent subsequent risks for morbidity and mortality among children born underweight (Black et al., 2008; Black et al., 2003; Victora et al., 1999).

Male sex was associated with increased risk of ALRI for children aged 0 – 23 months, and there are two possible explanations for this association, besides chance alone. One study from Bangladesh showed that male neonates were twice as likely as female neonates to be taken to a trained care provider during illness (Ahmed et al., 2001), so care seeking patterns could have caused this difference. However, we did not systematically collect data on who did not come to the clinic after referral so we cannot assess whether male children were more likely to seek care from these referrals. It is more likely that the association we observed reflects actual increased risk among males. Male infants are biologically more susceptible to more severe respiratory disease caused by viruses such as respiratory syncytial virus (Simoes, 2003), or human metapneumovirus (Williams et al., 2004), as well as pneumococcal disease (Klein, 1981), perhaps due to their shorter and narrower airways (Martinez et al., 1988).

A strength of our study is that we measured PM_2.5 concentrations to characterize exposure, which allowed us to quantify the levels of PM_2.5 associated with increased risk of respiratory illness. However, our measurements of exposure to PM_2.5 have limitations. Children were enrolled as they were born in the community from January 2008 through April 2009 but our PM measurements did not begin until May 2009. Exposure measurements do not temporally overlap with many of the ALRI episodes we observed, resulting in potential non-differential misclassification of PM exposure status which could bias our measure of association towards a null finding. Despite this, we observed a strong association between PM exposure and ALRI. A second limitation is that indoor measurements of PM_2.5 are proxy measurements for the actual exposure to PM_2.5 for study children. We observed an association for infants but not for children over a year old and this could be because once children begin to walk they spend less time indoors and, therefore, our PM_2.5 exposure measurement is a better indicator of exposure for infants than older children. It is also possible that our findings resulted from unmeasured confounders; however, we attempted to account for all confounders identified from similar studies. Finally, indoor PM_2.5 measurements were not adjusted for relative humidity and devices that use light scattering technology to measure PM_2.5 concentrations have been shown to overestimate PM_2.5 when relative humidity exceeds 85% (Quintana et al., 2000). Nevertheless, monthly average relative humidity in Dhaka ranged from 45% – 79% throughout the year (http://www.dhaka.climatemps.com, accessed 2011), so an overestimation of PM_2.5 in our study seems unlikely. In addition, all household PM_2.5 measurements would have been equally affected by days when relative humidity exceeded 85% so this would be unlikely to affect the associations we observed in our analysis.

Ninety percent of the mortality among children younger than 5 years of age in Bangladesh occurs in children <12 months of age (El Arifeen, 2008), and the leading cause of death in these children is ALRI (Black et al., 2003). Studies to identify risks associated with increased frequency of ALRI in younger age groups at higher risk for mortality may be useful for designing life-saving interventions (Katz et al., 2003). Our findings suggest that efforts to reduce indoor PM_2.5 exposures in low-income, urban Bangladeshi homes could meaningfully reduce ALRI morbidity and mortality. PM concentrations in our study households were high, including in households that used cleaner fuels for cooking. Interventions to reduce reliance on biomass for cooking in this community would reduce exposures, but additional research is needed to understand how to best reduce PM exposures in homes that already burn clean fuels.