There were 3,784 incident cases of DM among 74,412 eligible participants in the NHS (448 per 100,000 person-years) and 688 cases among 15,048 eligible participants in the HPFS (402 per 100,000 person-years) (). At baseline in 1989, the mean age was approximately 57 years for HPFS and 55 years for NHS participants. Most were never (NHS: 43%, HPFS: 45%) or former smokers (NHS: 36%, HPFS 46%). NHS participants were more likely than HPFS participants to have baseline BMI < 25 or > 30, although the prevalence of hypertension at baseline was similar for both groups. Men in the HPFS were more physically active and consumed more alcohol than did women in the NHS. The means and SDs of baseline particulate exposures for the HPFS and NHS were similar for PM2.5 [18.3 (3.1) and 17.5 (2.7) μg/m3], PM10 [28.5 (5.5) and 26.9 (4.8) μg/m3], and PM10–2.5 [10.3 (3.3) and 9.4 (2.9) μg/m3]. Baseline addresses for HPFS participants were more likely than NHS addresses to be 0–49 m from the nearest road (22.3% vs. 9.8%) and less likely to be ≥ 200 m from the nearest road (63.7% vs. 77.5%).
Descriptive characteristics of the NHS and HPFS participants at baseline, 1989.
Among the NHS, an interquartile range (IQR) increase of 4 μg/m3 in estimated PM10–2.5 averaged over the 12 months prior to diagnosis was associated with incident DM (HR = 1.07; 95% CI, 1.01–1.13) based on a single pollutant model stratified by age and adjusted for state of residence, year, and season (), but the HR was attenuated after additional adjustment for cigarette smoking, hypertension, BMI, alcohol intake, physical activity and diet (HR = 1.04; 95% CI, 0.98–1.10). Associations with IQR increases in PM2.5 (IQR: 4 μg/m3) and PM10 (IQR: 6 μg/m3) were similar to those estimated for PM10–2.5 in fully adjusted models (PM2.5 HR = 1.02; 95% CI, 0.94–1.09; PM10 HR = 1.03; 95% CI, 0.98–1.09), and associations with PM2.5 and PM10–2.5 were similar when both were included in the same model. Estimates did not change appreciably when time-varying rather than baseline BMI was included or when family history of diabetes, census tract median household value, or census tract median household income were added to models (data not shown). NHS participants living < 50 m from the nearest road were more likely to be diagnosed with DM than those living ≥ 200 m away (fully adjusted HR = 1.14; 95% CI, 1.03–1.27) (). A nonsignificant (p > 0.05) association was also evident for women with residences located 50–100 m from the nearest road.
HRs (95% CIs) for diabetes associated with an IQR increasea in average-predicted PM exposure in the 12 months before diagnosis.
HRs and 95% CIs for diabetes associated with proximity to nearest road.
Among the HPFS, estimated associations with DM were similar for IQR increases in PM2.5, (IQR: 4 μg/m3), PM10, (IQR: 7 μg/m3), and PM10–2.5 (IQR: 4 μg/m3) averaged over the previous 12 months (fully adjusted HR = 1.07; 95% CI, 0.92–1.24; 1.06, 95% CI, 0.94–1.20; and 1.04, 95% CI, 0.93–1.16, respectively) (). Estimated associations were similar when measures of neighborhood socioeconomic status (SES), time-varying rather than baseline BMI, and family history of diabetes were added to fully adjusted models (data not shown). Unlike the findings among women in the NHS with the fully adjusted multipollutant model, the risk of DM was greater for PM2.5 (HR = 1.06; 95% CI, 0.89–1.26) than PM10–2.5 (HR = 1.02; 95% CI, 0.89–1.16). In contrast with findings for the NHS, distance to the nearest road was not associated with incident DM among men in the HPFS ().
Pooled meta-analysis models adjusted for age, year, season, and state suggested a slight increase in diabetes incidence associated with an IQR increase in estimated PM10–2.5 and PM10 averaged over the 12 months prior to diagnosis (for both exposures: HR = 1.06; 95% CI, 1.01–1.12), although associations were attenuated after full adjustment (for both exposures: HR = 1.04; 95% CI, 0.99–1.09) (). Associations with PM2.5 were somewhat weaker. Tests of heterogeneity between the NHS and HPFS were not significant for any of the pooled meta-analysis models, but a multiplicative interaction term between sex and PM2.5 was statistically significant in a fully adjusted model (p = 0.04) (data not shown). The pooled meta-analysis of DM incidence and proximity to roadways indicated a statistically significant association with an address 0–49 m versus ≥ 200 m from the nearest road (HR = 1.11; 95% CI, 1.01–1.23) ().
Estimates from models adjusted for age, year, season, and state of residence, plus individual covariates were generally similar to estimates from fully adjusted models (data not shown). However, relative to basic models, adjusting for baseline BMI slightly attenuated associations with all size fractions of PM in the NHS (e.g., HR for PM2.5 in basic model adjusting for BMI = 1.02; 95% CI, 0.94–1.09) but slightly increased the HR for PM2.5 in the HPFS (HR = 1.08; 95% CI, 0.93–1.25). No evidence of effect modification was found for any of the covariates examined (data not shown).
Estimates from models that excluded participants with a baseline history of MI (leaving 3,672 NHS and 604 HPFS cases) or prior cancer (except nonmelanoma skin cancer) (leaving 3,311NHS and 522 HPFS cases) were similar to the reported estimates (data not shown). Compared with estimates from our original HPFS study population, analyses restricted to symptomatic DM cases (366) suggested a slightly lower, although still null, risk of incident DM with an IQR increase in PM exposures (fully adjusted PM2.5 HR = 0.96; 95% CI, 0.78–1.17; PM10 HR = 0.97; 95% CI, 0.82–1.15; PM10–2.5 HR = 0.98; 95% CI, 0.85–1.14). Results for symptomatic NHS participants (3,379) were comparable with those for all cases (data not shown).
In sensitivity analyses restricted to the first 2 years of follow-up, relative risks (RRs) for NHS women with IQR increases in all three particle fraction exposures were lower than for the full time period of follow-up, but higher when analyses were restricted to the final 2 years of follow-up (). Among men in the HPFS, associations were stronger with PM2.5 during the first 2 years and with all size fractions in the last 2 years than for the full period of follow-up, whereas the association with PM10–2.5 during the first 2 years was weaker.
HRs for diabetes associated with an IQR increasea before 12 months PM exposure for first and final 2 years of follow-up.
When associations were estimated with IQR increases in PM exposures averaged over the entire follow-up period (instead of during the 12 months prior to diagnosis), there was no evidence of associations between incident DM and any PM size fraction among NHS women, and associations with PM2.5 and PM10 were attenuated for HPFS men (data not shown). In pooled meta-analyses, HRs for incident DM associated with average PM exposures over the entire follow-up period were 0.99 (95% CI, 0.89–1.10) for PM2.5; 0.98 (95% CI, 0.91–1.06) for PM10; and 1.02 (95% CI, 0.93–1.12) for PM10–2.5.