Table 2.
Estimated associations between VOC groups and cardiovascular and asthma emergency department (ED) visits using three analytic approaches.a
| VOC GROUPS | INDICATOR POLLUTANT APPROACHb |
JOINT EFFECT ANALYSISc |
RANDOM EFFECT META-ANALYSISd |
|
|---|---|---|---|---|
| CARDIOVASCULAR ED VISITS AMONG ALL AGES | ||||
| HYDROCARBONS | ||||
| N-ALKANE | 1.002 (1.000, 1.005) | 1.006 (1.001, 1.011) | 0.999 (0.997, 1.001) | |
| ISO/ANTEISO-ALKANE | 1.004 (1.001, 1.008) | 1.005 (1.000, 1.010) | 1.000 (0.997, 1.003) | |
| OTHER ALKANE | 1.005 (1.001, 1.008) | 1.006 (1.001, 1.011) | 1.000 (0.997, 1.002) | |
| CYCLOALKANE | 1.005 (1.001, 1.009) | 1.005 (1.001, 1.010) | 1.002 (0.997, 1.007) | |
| ALKENE | 1.006 (1.002, 1.009) | 1.006 (1.002, 1.009) | 1.001 (0.997, 1.006) | |
| ALKYNE | 1.006 (1.003, 1.010) | 1.006 (1.003, 1.010) | 1.007 (1.001, 1.012) | |
| AROMATIC | 1.006 (1.002, 1.010) | 0.998 (0.992, 1.005) | 1.000 (0.999, 1.001) | |
| OXYGENATES | ||||
| ALDEHYDE | 1.001 (0.998, 1.004) | 1.008 (1.000, 1.016) | 1.000 (0.998, 1.002) | |
| ACID | 1.002 (0.995, 1.010) | 1.002 (0.995, 1.010) | 1.001 (0.993, 1.009) | |
| KETONE | 1.005 (0.995, 1.014) | 1.005 (0.995, 1.014) | 1.003 (0.993, 1.013) | |
| ASTHMA ED VISITS AMONG ALL AGES | ||||
| HYDROCARBONS | ||||
| N-ALKANE | 1.004 (1.000, 1.009) | 1.005 (0.995, 1.014) | 0.999 (0.994, 1.003) | |
| ISO/ANTEISO-ALKANE | 1.006 (1.000, 1.013) | 1.010 (1.000, 1.019) | 1.006 (0.999, 1.013) | |
| OTHER ALKANE | 1.006 (1.000, 1.013) | 1.007 (0.999, 1.016) | 0.999 (0.993, 1.004) | |
| CYCLOALKANE | 1.009 (1.002, 1.016) | 1.009 (1.002, 1.016) | 0.995 (0.986, 1.007) | |
| ALKENE | 1.005 (0.998, 1.011) | 1.005 (0.998, 1.011) | 0.992 (0.983, 1.003) | |
| ALKYNE | 1.006 (0.999, 1.012) | 1.006 (0.999, 1.012) | 1.000 (0.987, 1.014) | |
| AROMATIC | 1.009 (1.002, 1.017) | 1.008 (0.995, 1.021) | 1.002 (0.998, 1.005) | |
| OXYGENATES | ||||
| ALDEHYDE | 0.998 (0.991, 1.005) | 1.021 (1.004, 1.037) | 1.000 (0.995, 1.006) | |
| ACID | 1.008 (0.991, 1.026) | 1.008 (0.991, 1.026) | 1.003 (0.983, 1.021) | |
| KETONE | 1.026 (1.004, 1.048) | 1.026 (1.004, 1.048) | 1.024 (1.000, 1.049) | |
This analysis included 3224 days on which all VOCs were available during 8/14/1998–12/31/2008. VOC concentrations below the limit of detection (LOD) of 0.1 ppb-C were replaced with 0.05 ppb-C in all analyses. We used same-day (lag 0) pollution levels in models predicting cardiovascular ED visits and 3-day moving average (of lags 0, 1, and 2) pollution levels in models predicting asthma ED visits. All methods included the same covariate control for temporal trends and meteorology: time splines with monthly knots, cubic function of same-day maximum temperature, cubic function of lag 1–2-day moving average minimum temperature (when using 3-day moving average pollution levels), cubic function of mean dew point temperature (same-day or 3-day moving average, matching the temporal metric of the pollution term), day of week, indicators for holidays, seasons, season-maximum temperature interaction, season-day of week interaction, and indicators for hospital participation periods. The estimated associations are expressed as rate ratios (95% confidence interval) per interquartile range (IQR) increase in pollutant concentrations (listed in Table 1).
The “indicator pollutant approach” estimated the effect of each indicator pollutant increasing by its IQR in single-pollutant models.
The “joint effect analysis” estimated the effect of all pollutants in a group jointly increasing by their IQRs in multi-pollutant models that included all pollutants of the group. The joint effect estimates for VOC groups comprised of only one pollutant were the same as the estimates obtained from the indicator pollutant approach.
The “random effect meta-analysis” estimated the mean effect of any of the pollutants in a group increasing by its IQR in a two-stage regression, where the 46 individual pollutant effects were estimated simultaneously in the Poisson model in the first stage, and the mean of each group was estimated under a Bayesian framework using Markov chain Monte Carlo in the second stage. The estimated rate ratio (95% CI) for the random effect meta-analysis is median (2.5th, 97.5th percentiles) from the posterior distribution.