The study included 488 (40.2%) male subjects and 725 (59.8%) female subjects between 11 and 84 years of age (mean age = 43.5 years) (). Only eight subjects were < 18 years of age. Fasting plasma tHcy ranged between 2.9 and 59.6 μmol/L, with a mean concentration of 9.0 μmol/L (95% CI, 8.8–9.2). PML tHcy ranged between 11.0 and 83.7 μmol/L, with a mean concentration of 24.2 μmol/L (95% CI, 23.8–24.6). Both fasting and PML tHcy exhibited a positive association with female sex, cigarette smoking, alcohol, and body mass index (BMI) (). Older subjects had higher fasting plasma tHcy, but PML tHcy was not significantly associated with age. To evaluate possible confounding effects, we generated linear regression models that included sex, cigarette smoking, alcohol, BMI, and age as independent variables. These models showed an independent effect of sex (p < 0.001 on fasting plasma tHcy; p < 0.001 on PML tHcy) and smoking (p = 0.03 on fasting plasma tHcy; p = 0.002 on PML tHcy), but no significant associations were found for age (p = 0.07 for fasting plasma tHcy; p = 0.19 for PML tHcy), alcohol (p = 0.30 for fasting plasma tHcy; p = 0.92 for PML tHcy), or BMI (p = 0.91 for fasting plasma tHcy; p = 0.49 for PML tHcy)
Fasting and postmethionine-load total homocysteine levels, by study subjects’ characteristics.
We estimated air pollution exposure on the basis of ambient measurements taken during the study period. Air pollution levels and weather variables in the study area from 1 January 1995 to 1 September 2005 are summarized in .
Air pollution profile and weather variables in Lombardia Region, Italy, from 1 January 1995 to 1 September 2005.a
presents the estimated mean differences of fasting and PML tHcy associated with an IQR increase in the average concentrations of air pollutants in the 24 hr or in the 7 days before the study. The estimates were adjusted for age, sex, body mass index, cigarette smoking, alcohol consumption, and hormone use. In addition, penalized smoothing splines were used in the models to adjust for nonlinear effects of day of the year, long-term time trend, and temperature.
Estimates of the effect of IQR increase in air pollutants on fasting and postmethionine-load homocysteine.
PM10 levels did not show a significant associations with fasting and PML tHcy. The estimated increase in tHcy associated with the average PM10 concentrations in the 24 hr before the study was 0.4% (95% CI, −2.4 to 3.3) for fasting and 1.1% (95% CI, −1.5 to 3.7) for PML tHcy (differences were not statistically significant). Increases in 7-day PM10 levels were associated with nonsignificant 1.0% (95% CI, −1.5 to 3.7) and 2.0 (95% CI, −0.6 to 4.7) increases in fasting and PML tHcy, respectively. Among the gaseous pollutants, the average concentrations of O3 in the 24 hr before the study were significantly associated with a 6.7% (95% CI, 0.9–12.8) increase in fasting tHcy, but the association with PML tHcy was not significant (3.6%; 95% CI, −1.4 to 9.0). No association was found between the 7-day O3 average levels and fasting or PML tHcy. Similarly, the other gaseous pollutants were not associated with tHcy levels ().
We then evaluated the relation between air pollution exposure, smoking, and plasma tHcy levels. Among smokers, 24-hr PM10 levels were associated with 6.3% (95% CI, 1.3–11.6; p < 0.05) and 4.9% (95% CI, 0.5–9.6; p < 0.05) increases in fasting and PML tHcy, respectively (). In nonsmokers, the estimated differences were −1.7% (95% CI, −4.8 to 1.5) for fasting and −0.3 (95% CI, −3.1 to 2.6) for PML tHcy. The tests for interaction between PM10 and smoking were statistically significant (p = 0.005 for fasting tHcy; p = 0.039 for PML tHcy), whereas smoking in such models was not associated per se with differences in fasting (main effect = −5.9%; 95% CI −14.1 to 3.2; p = 0.20) or PML tHcy (main effect = −0.6%; 95% CI −8.5 to 8.0; p = 0.89). The association between the PM10 levels in the 7 days before the study and tHcy showed the same pattern. For smokers, 7-day PM10 was associated with a nonsignificant 3.3% (95% CI, −1.5 to 8.4) increase in fasting tHcy and a significant 5.2% (95% CI, 0.8 to 9.8; p < 0.05) increase in PML tHcy. Although no significant increase in tHcy was found among non-smokers (0.1%, 95% CI, −3.2 to 3.3 for fasting tHcy; 0.7%, 95% CI, −2.2 to 3.7 for PML tHcy), the tests for interaction between average 7-day PM10 and smoking were not statistically significant (p = 0.23 for fasting tHcy; p = 0.07 for PML tHcy).
Estimates of the effect on fasting homocysteine of an IQR increase in the average concentration of PM10 during the 24 hr before the study, by cigarette smoking. *p < 0.05.
Our main results for PM10 exposure reported above were based on analysis that included both measured PM10 and predicted PM10 values that were used for the earlier time periods in which TSP rather than PM10 data were available (see “Materials and Methods”). The results based only on measured PM10 confirmed the presence of a statistical interaction between PM10 and smoking. Among smokers, measured 24-hr PM10 levels were associated with 6.2% (95% CI, 0.0–12.7; p < 0.05) and 6.0% (95% CI, 0.5–11.8; p < 0.05) increases in fasting and PML tHcy, respectively. In non-smokers, the estimated differences were −1.6% (95% CI, −5.5 to 2.5) for fasting and −0.1 (95% CI, −3.6 to 3.5) for PML tHcy. Again, the tests for interaction between measured PM10 and smoking were statistically significant (p = 0.026 for fasting tHcy; p = 0.048 for PML tHcy). As for the main analysis based on both measured and predicted PM10 levels, measured mean PM10 levels in the 7 days before the study were associated with higher tHcy in smokers but not in nonsmokers, whereas the interaction terms were again not statistically significant (data not shown).
Gaseous pollutants (CO, NO2, SO2, and O3) showed no significant interaction with cigarette smoking in association with the levels of fasting (p > 0.26) and PML tHcy (p > 0.43). In addition, no significant interaction of the air pollutants evaluated with age, sex, alcohol use, overweight (BMI > 25), and obesity (BMI > 30).