Using mixed effect linear regression, we modeled the impact of fluoridation status on the difference in arsenic concentrations between water entering the drinking water system and treated water (treated minus raw) while controlling for multiple factors (Table 2). Fluoridated systems were associated with an additional 0.078 μg/L (95% CI 0.021, 0.136) of arsenic when compared to non-fluoridated systems (P = 0.008) while controlling for raw water concentrations, treatment processes (coagulation, microfiltration, ultrafiltration), and source water (ground vs. surface). Also, for every 1 μg/L increase in raw water arsenic levels, the treatment process is associated with the removal of 0.55 μg/L of arsenic on average while controlling for all other model variables.
Results of the mixed effect linear regression analysis
| Parameter . | Estimate (95% CI) . | P-value . |
|---|---|---|
| Fluoridation present | 0.078 (0.021, 0.136) | 0.008 |
| Raw water arsenic (per 1 μg/L increase) | −0.552 ( − 0.597, −0.506) | <0.001 |
| Microfiltration present | −0.140 ( − 0.307, 0.027) | 0.099 |
| Ultrafiltration present | 0.024 ( − 0.104, 0.153) | 0.707 |
| Surface water (ground as reference) | 0.101 ( − 0.046, 0.248) | 0.177 |
| Coagulation present | −0.071 ( − 0.273, 0.130) | 0.484 |
| Interaction between Ground/Surface and Coagulation | −0.260 ( − 0.506, −0.013) | 0.039 |
| Parameter . | Estimate (95% CI) . | P-value . |
|---|---|---|
| Fluoridation present | 0.078 (0.021, 0.136) | 0.008 |
| Raw water arsenic (per 1 μg/L increase) | −0.552 ( − 0.597, −0.506) | <0.001 |
| Microfiltration present | −0.140 ( − 0.307, 0.027) | 0.099 |
| Ultrafiltration present | 0.024 ( − 0.104, 0.153) | 0.707 |
| Surface water (ground as reference) | 0.101 ( − 0.046, 0.248) | 0.177 |
| Coagulation present | −0.071 ( − 0.273, 0.130) | 0.484 |
| Interaction between Ground/Surface and Coagulation | −0.260 ( − 0.506, −0.013) | 0.039 |
The outcome of the model is treated minus raw arsenic concentrations in μg/L. n = 1,329.