Effect of pollutants on respiratory hospitalizations, main results—children aged 2–5, using a single index for pollution instead of individual components
| Index used: | First principal component: weighted average (of standardized components) | Simple average of standardized components | First principal factor: weighted average (of standardized components) | |||
|---|---|---|---|---|---|---|
| Controls: | Basic | Family fixed effects and quarterly weather | Basic | Family fixed effects and quarterly weather | Basic | Family fixed effects and quarterly weather |
| Panel A Basic linear specification | ||||||
| Index | 0.0021** [0.0010] | 0.0034** [0.0013] | 0.0055** [0.0023] | 0.0082*** [0.0028] | 0.0033** [0.0016] | 0.0054** [0.0025] |
| Observations | 114612 | 114612 | 114612 | 114612 | 114612 | 114612 |
| R-squared | 0.282 | 0.501 | 0.282 | 0.501 | 0.282 | 0.501 |
| Panel B Nonparametric estimates (bottom quartile of index serves as reference group) | ||||||
| Second quartile = 1 | 0.0022 [0.0025] | −0.0020 [0.0046] | 0.0037 [0.0026] | −0.0014 [0.0047] | 0.0024 [0.0021] | 0.00056 [0.00276] |
| Third quartile = 1 | 0.0056 [0.0036] | 0.0024 [0.0051] | 0.0057** [0.0032] | 0.0020 [0.0050] | 0.0060** [0.0029] | 0.0065** [0.0032] |
| Fourth quartile = 1 | 0.0062 [0.0042] | 0.0067 [0.0056] | 0.0076** [0.0038] | 0.0056 [0.0052] | 0.0048* [0.0026] | 0.0043 [0.0035] |
| Observations | 114,612 | 114,612 | 114,612 | 114,612 | 114,612 | 114,612 |
| R-squared | 0.282 | 0.501 | 0.282 | 0.501 | 0.282 | 0.501 |
Robust standard errors in brackets. The specification with basic controls includes all the regressors in Column 1 of Table 4, and the specification with family fixed effects and quarterly weather includes all the regressors in Column 4 of Table 4. The indexes are constructed as weighted averages of the five major pollutants, after each pollutant has been standardized to have mean 0 and standard deviation of 1. The different indexes are computed as follows:
First principal component = 0.5161*PM10 + 0.4973*CO + 0.2362*O3 + 0.5296*SO2 + 0.3873*NO2. Mean 0, standard deviation 1.41 Simple average = (PM + CO + 03 + S02 + N02)/5. Mean 0, standard deviation 0.62 First Principal factor = 0.6294*PM10 + 0.6041 *CO + 0.2198*O3 + 0.5208*S02 + 0.3966*NO2. Mean 0, standard deviation 0.812
↵* significant at 10 percent; ** significant at 5 percent; *** significant at 1 percent