Introduction

The Regional Greenhouse Gas Initiative (RGGI) is the first mandatory cap-and-trade programme designed to reduce greenhouse gas (GHG) emissions in the USA. Cap-and-trade refers to creating a limit or ‘cap’ on GHG emissions from various emission sources. The RGGI programme limits carbon dioxide (CO₂) emissions in particular from the electric power sector, the largest source of CO₂ emissions for more than 40 years in the USA. The RGGI programme is often referred to as a market-based CO₂ emission reduction programme because, under this programme, the marketplace determines the economically efficient solution for GHG emission reduction. The RGGI programme became effective in January 2009 in 10 northeastern states. New Jersey originally participated in this programme in 2009, but withdrew in 2011. Accordingly, electric power plants with 25 or more megawatts capacity in the remaining nine states continue to participate in the RGGI programme. These nine states are Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New York, Rhode Island and Vermont. The RGGI programme consists of four compliance periods, each of which lasts for 3 years (ie, 2009–2011, 2012–2014, 2015–2017 and 2018–2020). During the first 3-year compliance period (2009–2011), the emission cap was 188 million short tons of CO₂ (mtCO₂) per year, which was decreased to 165 mtCO₂ during the next period (2012–2013). The RGGI states then substantially reduced the emission cap from 165 mtCO₂ to 91 mtCO₂ in 2014. This cap has then gradually decreased by 2.5% each year from 2015 until 2020. As expected, CO₂ emissions have fallen considerably in these nine states since its implementation in 2009.1–4

Prior studies have consistently shown an association between high concentrations of ambient air pollutants and an increased risk of morbidity and premature mortality. For example, extensive evidence has shown that ambient air pollutants were positively associated with asthma, lung cancer, respiratory mortality, cardiovascular mortality and total mortality.5–8

Some studies examined the impact of air pollution particularly on infant mortality9–11; however, the findings from these studies are inconsistent. One study found that regulations to reduce air pollution led to modest but statistically insignificant improvement in infant mortality in India9 whereas other studies showed that infant mortality was significantly lowered following environmental regulations in Germany and China.10 11 The variations in the findings across studies might be driven by the different strengths of the relationships between air pollution and infant mortality among the countries.

In the USA, some studies have shown that reductions in air pollution significantly decreased infant mortality.12–14 These studies evaluated infant mortality in the 1990s or earlier. Recent US-based studies regarding infant mortality have focused largely on its association with factors other than air pollution such as infants’ demographics,15–17 state-level minimum wage laws18 and Medicaid expansion.19 The objective of this study was to investigate the impact of the environmental policy, RGGI, on infant mortality in recent years in the USA. Because some studies suggested that the effects of air pollution on health outcomes may be sex-specific,20–22 we examined its impact also by sex.

Methods

Results

Characteristics of RGGI states and non-RGGI states are presented in table 1. Compared with non-RGGI states, RGGI states had lower total number of births, number of hospitals and average temperatures although the level of the median household income and the average birth weight in RGGI states were higher. However, no significant differences were observed between RGGI states and non-RGGI states in characteristics such as GDP, residential population, birth rate of black babies and mother’s education levels. In addition, the sex ratios of newborn males to newborn females were also similar between RGGI states and non-RGGI states before RGGI implementation (1.051 and 1.050, respectively) and after RGGI implementation (1.053 and 1.049, respectively) (data not shown in table 1).

Figure 1 shows a trend in the observed overall NMRs and overall IMRs over time. Prior to RGGI implementation (2003–2008), changes in overall NMRs were –0.23 (=4.38–4.61) per 1000 live births and –0.38 (=4.17–4.55) per 1000 live births in non-RGGI states and RGGI states, respectively. Accordingly, RGGI states had a larger change in overall NMRs during 2003–2008 (difference in NMR changes between RGGI states and non-RGGI states during 2003–2008=0.15 per 1000 live births). After RGGI implementation (2009–2014), changes in overall NMRs were –0.13 per 1000 live births and –0.59 per 1000 live births in non-RGGI states and RGGI states, respectively. Therefore, we could observe a much larger decrease in overall NMRs in RGGI states compared with non-RGGI states after RGGI implementation. The difference in NMR decreases between RGGI states and non-RGGI states during 2009–2014 (0.46 per 1000 live births) was about three times higher than that before RGGI implementation (0.15 per 1000 live births). Similarly, the decrease in overall IMRs among RGGI states was larger than that among non-RGGI states after RGGI implementation.

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Figure 1

Observed (A) overall NMRs and (B) overall IMRs in RGGI states and non-RGGI states in the USA, 2003–2014. IMR, infant mortality rate; NMR, neonatal mortality rate; RGGI, Regional Greenhouse Gas Initiative.

Table 2 presents the DID estimates of the RGGI’s impact on mortality outcomes. As noted earlier, an interaction term in the adjusted DID model captures the effect of RGGI on infant death rates in RGGI states compared with its effect in non-RGGI states. Our results from an adjusted DID model show that implementation of RGGI was associated with significant decreases in overall NMRs (a reduction of 0.41/1000 live births) and male NMRs (a reduction of 0.43/1000 live births) in RGGI states (p<0.001 and p=0.016, respectively). However, RGGI did not have a significant effect on female NMRs (p=0.086). Similarly, overall IMRs and male IMRs decreased significantly by 0.37/1000 live births and 0.61/1000 live births, respectively, after implementation of RGGI (p=0.003 and p=0.001, respectively) while female IMRs were not significantly affected by RGGI (p=0.068). The full results obtained from the DID regressions are available in online supplementary table 1.

These findings were not substantially changed in our sensitivity analysis when data from the years 2008 and 2009 were excluded in the analysis (table 3). That is, overall NMRs and male NMRs (0.53/1000 live births and 0.62/1000 live births, respectively) as well as overall IMRs and male IMRs (0.51/1000 live births and 0.85/1000 live births, respectively) were significantly reduced in RGGI states after the implementation of RGGI. However, female IMRs became significant with a reduction of 0.35/1000 live births in the sensitivity analysis.

After verifying significant decreases in infant mortality outcomes among RGGI states during 2009–2014, we conducted further analyses to examine the association between an air pollutant (CO₂) and infant mortality. Our results indicate that the concentration of atmospheric CO₂ was positively associated with infant mortality (change in overall NMRs per unit increase in CO₂: 0.007/1000 live births, p=0.016; change in overall IMRs per unit increase in CO₂: 0.009/1000 live births, p=0.023, see online supplementary table 2 for full results).

Discussion

RGGI, the first market-based regulatory GHG emission trading scheme (ETS) in the USA, was implemented in 2009. The objective of this study was to examine whether its implementation affected public health, particularly infant death rates, since infant mortality has been widely accepted as a proxy for public health. Our results showed that the implementation of RGGI was associated with a reduction in overall neonatal deaths and overall infant deaths. This could be explained by fuel switching trends observed in RGGI states. Previous studies have reported that RGGI implementation has encouraged fuel switching from coal to clean natural gas in electricity generation in the USA.2 3 28 Fossil fuels such as coal and petroleum are high-carbon intensity sources which emit two times or more CO₂ than does natural gas. As such, total CO₂ emissions from power plants in RGGI states have dropped substantially. A decrease in air pollutant (CO₂) concentrations in RGGI states might be associated with the reduced overall infant death rates in these states. Associations of air pollution with risk of premature mortality have been evidenced in a number of studies.7 8 10 11 Similarly, our further analyses also showed that overall NMRs and IMRs increased as the CO₂ level increased. Air pollution’s adverse effects on fetal development were suggested by Chay and Greenstone as a mechanism through which air pollution affects infant mortality.13 14

Another finding of this study was the differences in the impact of RGGI on the infant health of males and females. That is, male NMRs and male IMRs were significantly reduced after the implementation of RGGI while this environmental policy did not significantly affect deaths in female infants. This finding aligns with the traffic-related air pollution on childhood asthma (TRAPCA) study reporting different health responses in boys and girls aged 0–2 years to air pollution.29 The TRAPCA study showed stronger associations between air pollutants and non-specific respiratory symptoms (eg, cough without infection, nocturnal dry cough) among males than among females.29 Greater susceptibility to diseases and higher mortality among boys are used to explain by the differences in genetic and biological makeup between boys and girls. Female infants are known to have larger airways relative to body size30 and their specific airway resistance is lower than in male infants.31 However, studies of older children and adults often suggest stronger effects of air pollution among females. For older-aged children and adults, not only biological sex differences but also other social gender differences come into play in modifying the effects of air pollution. In general, social gender can affect the presence of exposures (eg, exposure from using cosmetics or jewellery), the intensity of exposures (eg, frequency of cooking) and coexposures (eg, diet or smoking). These social gender differences can change over the life course and do not exhibit predictable patterns. Therefore, these social gender differences aggregated with biological sex differences make gender effects more complicated, which might have led to large variations in the findings among prior studies for older-aged children and adults.21 Regarding infants in this study and the TRAPCA study, observed modification was most likely attributable to biological sex differences. In other words, social gender differences were less likely to serve as an effect modifier in these two studies.

The impacts of implementation of RGGI on infant death rates were not changed substantially when the timeframe was varied in our sensitivity analysis. We excluded data from the years 2008 and 2009 in our sensitivity analysis to isolate the impact of RGGI on infant deaths while eliminating the effects of the economic recession and the moderate weather experienced during this time period. As noted earlier, the economic recession and moderate weather could have led to a reduction in GHG emissions, thereby affecting infant mortality. Findings in the sensitivity analyses still indicated a linkage between RGGI and infant mortality reduction (eg, total NMRs and male NMRs as well as total IMRs and male IMRs). Notably, female IMRs also reduced after the RGGI implementation although its impacts on female IMRs were much smaller than those on male IMRs.

To the best of our knowledge, this study represents the first attempt to examine infant health effects associated with a specific US environmental regulation, RGGI. In particular, we examined the RGGI effects on infant health stratified by sex. Our study findings have important implications for environmental and public health policy. According to the WHO, one out of every nine deaths at all ages directly resulted from air pollution in 2012, indicating air pollution as a large environmental health risk.32 The annual global cost of healthcare related to pollution was estimated to be between $240 billion and $630 billion.33 To cope with air pollution problems, the European Union implemented the European Union (EU) ETS in 2005, which is the world’s first cap-and-trade programme. Benefits of this programme have emerged yielding significant reductions in GHG emissions in the EU.34 35 Similarly, our findings show the beneficial effects of the USA’s cap-and-trade programme to limit CO₂ emissions, which informs policymakers of the tangible public health benefits of RGGI. RGGI’s experiences can encourage non-RGGI states to enhance their efforts to reduce CO₂ emissions or participate in the RGGI programme, thereby broadening the RGGI market. This could lead to air quality improvements in more states, which can significantly benefit infant health. Alternatively, RGGI could be viewed as potential federal legislation.

Several limitations warrant discussion. First, an assumption behind the DID approach is that without the implementation of RGGI, the non-RGGI states would have experienced a similar trend in infant deaths to the RGGI states (parallel trend assumption). However, this assumption could be violated if there is any unobserved policy change affecting infant health in non-RGGI states only. A violation of the parallel trend assumption would make our estimates biased. As such, we performed a test to check whether this assumption was met. Our test results of the pre-RGGI period showed no significantly different trends in infant mortality between RGGI states and non-RGGI states. While this trend might not persist in the post-RGGI period, it suggests that any bias from the violation of this assumption is likely to be small. Second, RGGI states have entered a second phase in which the existing cap level has been reduced to match their actual CO₂ emissions levels since 2014. Accordingly, the emission cap declines 2.5 percent each year between 2015 and 2020 in this new phase. Therefore, it is uncertain how differently the adjusted (reduced) emissions cap will impact infant mortality than the original emissions cap. A future follow-up study extending the timeframe may elucidate this point. Likewise, our study findings may not be generalizable to a GHG reduction policy outside the USA. Moreover, infant deaths in this study were all-cause mortality rather than disease-specific mortality. Accordingly, our analyses were not granular enough to capture the reasons for deaths. Condition-specific mortality would provide information on which fetal organ systems are most affected by air pollution. However, despite this limitation, our study shows the comprehensive health effects in infants associated with air pollution. Finally, residual confounding by unmeasured factors might be possible, which is the limitation inherent in any observational studies. The study findings could be biased to the extent unmeasured confounders affect the association of RGGI implementation with infant mortality.

In conclusion, the RGGI programme started as a means to limit CO₂ emissions in January 2009 in the USA. Although this programme was originally designed to spur innovation in the clean energy economy, it also conferred significant benefits to infant health. Our study found that RGGI was associated with decreases in infant mortality. This finding would be informative to policymakers. Future environmental and public health policy can be implemented toward expanding the benefits of RGGI.