Frequently asked questions

The aim of this study was to investigate possible health effects associated with (i) Municipal Waste Incinerator (MWI) emissions of particulate matter ≤10 µm in diameter (PM10) as a proxy for municipal waste incinerator emissions more generally, and (ii) living near a municipal waste incinerator, in relation to fetal growth, stillbirth, infant mortality, congenital anomalies and other birth outcomes.

Incineration is the process of burning waste to dispose of it. An incinerator is a furnace where waste is burnt. There are many types of incinerators; this study concerns Municipal Waste Incinerators (MWIs). Municipal waste incinerators (MWIs) are used to burn residual municipal solid waste, which is waste that comes from households after recycling, reuse or composting and that would otherwise be sent to a landfill. MWIs typically operate at approximately 850°C, and are fitted with abatement technologies to reduce the concentrations of pollutants emitted from them. All UK MWIs are designed to recover energy by using the heat to generate steam to drive turbines for electricity, and some also provide heat to the local area. Emissions are tightly regulated to keep these pollutants at low level, with regular monitoring carried out and evaluated by the Environment Agency. 

In the SAHSU studies, we found extremely low exposures at ground-level from MWI particulate emissions – these were 1/100 to 1/10,000 of background levels of ambient particulate air pollution.  (Douglas et al 2017)   .  We found that metals from MWI emissions could not be detected at ground level or were detected rarely – we looked at six MWIs with air quality monitoring stations nearby measuring metals in air.  We did not detect ground level metals characteristic of incineration for four of six incinerators and for the other two it was 0.2% and 0.1% of the time. Further, we were able to calculate contribution of MWI emissions to the ambient air levels of Cadmium and Chromium and these were very small (ranging from 0.001% to 0.08%) (Font et al., 2015). We were unable to directly assess dioxins and similar compounds but deposition of particulate emissions was taken to be representative of exposure to other components of incinerator emissions. We found that the spatial spread of PM10 from MWIs is also likely to reflect the exposure patterns of other gaseous emissions (such as sulphur dioxide (SO2) and nitrogen oxides (NOx) from MWIs (see Ghosh et al., 2018, supplement C).

As with any combustion process, the main pollutant emitted from MWIs is oxides of nitrogen (NOx). In addition, MWIs emit small amounts of particulate matter, sulphur dioxide (SO2), hydrogen chloride (HCl), carbon monoxide (CO), volatile organic compounds (VOCs), heavy metals and persistent organic pollutants (POPs), such as polychlorinated dibenzo-p-dioxins/furans (PCDD/Fs), polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs). Emissions from MWIs are strictly controlled by their environmental permits. This study focuses on PM10 emissions and concentrations.

The use of incineration for waste disposal in the UK is increasing due to restrictions on the use of landfill. The incineration process and emissions from Municipal Waste Incinerators are tightly regulated, however there is public concern over possible health risks and this study was commissioned to extend the evidence base and provide more information to the public on this subject.

The study found that living near an incinerator and being exposed to emissions from an incinerator were not associated with risk infant death or stillbirth, low birthweight, preterm delivery, multiple births (twins, triplets etc.) or the baby’s sex (Ghosh et al., 2018). A further paper looked at birthweight and infant mortality before and after a new MWI opened.

For congenital anomalies, no increased risk was found in relation to exposure to emissions from MWIs (see “What is a dispersion model and can we rely on the results?”), but small increased risks of  2% for all congenital anomies combined and up to 7% for genital anomalies  were observed for living near an MWI for all congenital anomalies combined, congenital heart defects and genital anomalies, specifically hypospadias. These findings in proximity to MWI might reflect residual confounding (see “What does “residual confounding” mean and what were the limitations of the confounding factors used in the study on congenital anomalies?“), although it is not possible from these data to exclude a potential causal effect even in the absence of associations with modelled emissions.

When you are looking to see if there is any relationship between an independent variable (in this case either exposure to emissions from an MWI or proximity to an MWI) and a dependent variable (in this case congenital anomalies), you also need to consider whether there could be any other factors which could be associated with both variables and which could distort the results. For example, higher levels of deprivation are known to lead to higher numbers of congenital anomalies, and so the level of deprivation in the area an MWI is located in will be an influencing factor on the results and will need to be taken into account. Such factors are also known as “confounding factors” or “confounders”, and the process of taking them into account is known as “confounding”.

In plain terms, a confounding factor for the purposes of the study on congenital anomalies was any factor that could have an influence on congenital anomalies, other than the ones which the study was testing i.e. exposure to emissions and proximity to the MWI.

The confounding factors that were taken into account can be divided into those at the individual level (i.e. known for the individual mother) and the area level (i.e. known for the area in which the mother lived). Individual level confounders were maternal age and year of birth or termination. Area-level confounders were deprivation, area-level ethnicity, major road density and other sources of emissions.

One of the conclusions of the study is that the “findings in proximity to MWI might reflect residual confounding”. What this means is that there may be other compounding factors which were not possible to take into account (e.g. because no data exists for them or it would be too difficult or time-consuming to generate such data) but which may nevertheless have had an influence on the rate of congenital anomalies and explain the results obtained in the study.

All of the area-level confounders (see above) were best available information, but also limited in that they did not apply to individual mothers, i.e. just because a mother lives in an area with a high level of deprivation does not necessarily mean she herself is experiencing the same high level of deprivation, and vice-versa.

Road density based on the length of motorways, A-roads and B-roads also only gives an approximate indication of the relative level of transport emissions that may arise in a particular area, as they do not into account the amount of traffic that actually uses those roads, nor the density of any smaller roads in that area.

The confounding factor for other sources of emissions was also limited in that it only took into account emissions [reported above the reporting threshold] for larger industrial processes regulated by the Environment Agency, Natural Resources Wales or Scottish Environmental Protection Agency, and therefore did not capture any smaller industrial processes regulated by local authorities (or that do not require an environmental permit).

The following factors may explain the associations detected with distance:

• Emissions from waste handling including waste transportation to the MWI
• Incomplete accounting for emissions from traffic and industry in the area due to the limitations of the dataset used
• Using deprivation and ethnicity data for the area rather than for individual mothers (residual confounding)
• Ambiguity on the coding of diseases under the International Classification of Diseases system
• Inconsistent reporting of the severity of certain anomalies across registries and differences between English and Scottish datasets
• Reporting systems in place that were unlikely to provide effective surveillance of hypospadias.

No. The study does not say that the small excess risks associated with congenital heart disease and genital anomalies in proximity to MWIs are caused by those MWIs, as these results may be explained by residual confounding factors i.e. other influences which it was not possible to take into account in the study. This possible explanation is supported further by the fact that the study found no increased risk in congenital anomalies due to exposure to emissions from incinerators.

No increased risk was observed for any of the health effects investigated in relation to exposure to the emissions from MWIs.

The small increased risk of congenital anomalies observed in proximity to MWIs may well reflect residual confounding, although it is not possible to completely exclude a possible causal effect even in the absence of associations with modelled emissions.

The findings of the study provide support for the current PHE position statement “While it is not possible to rule out adverse health effects from modern, well-regulated municipal waste incinerators with complete certainty, any potential damage to the health of those living close-by is likely to be very small, if detectable. This view is based on detailed assessments of the effects of air pollutants on health and on the fact that modern and well managed municipal waste incinerators make only a very small contribution to local concentrations of air pollutants”. (https://www.gov.uk/government/publications/municipal-waste-incinerators-emissions-impact-on-health).

The odds ratio (OR) is a measure of association between exposure and outcome, such that an odds ratio of 1.03 can be interpreted as an increase in risk of 3%. The 95% confidence interval (95%CI) is used to quantify the uncertainty in the estimates of odds ratios and is usually interpreted that we are 95% certain that the true odds ratio is with the 95%CI.

In the UK, congenital heart defects affect approximately 5.3 in 1000 births and 1.9 per 1000 males are born with hypospadias ( NCARDRS, 2016). In terms of excess risk, the team estimates that the associated increase in risk for these two birth defects could be around 0.6 cases per 1,000 total births for congenital heart defects and 0.6 cases per 1,000 male births for hypospadias within 10 km of an incinerator. The study as a whole had 1,232 cases of congenital heart disease and 407 cases of hypospadias.

Hypospadias and congenital heart defects typically require surgery but are rarely life-threatening. Note that we found no association with proximity to nearest MWI when specifically analysing severe congenital heart defects.

All 22 municipal waste incinerators operating in Great Britain between 2003 and 2010 were included for the majority of the health effects considered. The sites were: Allington, Bolton, Chineham, Coventry, Crymlyn Burrows, Dudley, Dundee, Eastcroft, Edmonton, Grundon(Lakeside), Isle of Wight, Kirklees, Marchwood, Newlincs (Grimsby), Porthmellon, Portsmouth, SELCHP, Sheffield, Stockton-on-Tees, Stoke-on-Trent, Tyseley, Wolverhampton.

For the investigation into risks of congenital anomalies, only the 10 municipal waste incinerators located in areas covered by a local congenital anomaly register could be included. These were: Chineham, Dundee, Eastcroft, Isle of Wight, Marchwood, Newlincs (Grimsby), Porthmellon, Portsmouth, Sheffield, Stockton-on-Tees. Other areas did not have complete information on congenital anomaly cases.

The 10 km distance was chosen for consistency with screening criteria used for implementing the Habitats Regulations: incineration plants that are within 10 km of a European Site require an assessment of their impact for short-range air emissions, so this was taken as the likely range of impact on the local environment. (See Ashworth et al., 2013)

Over 1 million births between 2003 and 2010 were included in the study of 22 incinerators in Great Britain. For the investigation into risks of congenital anomalies, approximately 220,000 births were included in the study.

The health outcome results given in the published papers were from pooled data across all the incinerators included in the study. The numbers of births around individual incinerators were not large enough to produce meaningful (i.e. statistically significant) results.

Both ethnicity and deprivation are associated with higher risks for some of the outcomes studied, independent of any potential effects from MWI exposures, so we included an adjustment for these factors in our analyses.

(References for effects of ethnicity and deprivation:

van der Zanden LFM, van Rooij I, Feitz WFJ, Franke B, Knoers N, Roeleveld N. Aetiology of hypospadias: a systematic review of genes and environment. Human Reproduction Update 2012; 18(3): 260-83.

Varela M, Nohr EA, Llopis-Gonzalez A, Andersen AMN, Olsen J. Socio-occupational status and congenital anomalies. European Journal of Public Health 2009; 19(2): 161-7.)

This is the largest study to date to examine potential impacts of modern MWIs operating to current EU regulations on a range of outcomes. Other studies have used distance as a measure of exposure or modelled emissions (either particulate air pollution less than 10 microns in diameter (PM10) or another airborne pollutant) but this was the first study to look at both distance and modelled emissions. Results from most studies were consistent with ours, although two studies Candela et al. (2013) and Sontoro et al. (2016) found associations between preterm births and MWI related PM10 exposure, which we did not. Of the previous investigations of municipal waste incineration and health specifically focussed on congenital anomalies, the start dates of all studies pre-date the implementation of the Industrial Emissions Directive for existing MWIs (28 December 2005) so emissions levels are likely to have been higher.

Five separate studies have found some increased risks with specific congenital anomaly groups, including: facial clefts (Cordier et al., 2004, ten Tusscher et al., 2000); renal and urological anomalies (Cordier et al., 2004; 2010); neural tube defects, spina bifida and lethal congenital heart defects (CHDs) (Dummer et al., 2003); and deaths due to all congenital anomalies combined (Tango et al., 2004). We did not find associations with these anomalies, but with genital deformities and congenital heart anomalies with distance, but not with emissions (see “What did the study find regarding reproductive and infant health?”). However, other studies have found no associations between MWIs and congenital anomalies for example Vinceti et al. (2008; 2009).

Yes, the same emissions standards are in place today as in 2003-10. Municipal waste incinerator emissions standards in Great Britain were previously set by the European Union Waste Incineration Directive (EU-WID)(2000/76/EC), which came into operation for new and existing MWIs on 28 December 2002 and 2005, respectively. Some of the older MWIs were potentially operating to different standards at the start of the study, but from end 2005 all were operating to EU-WID standards. Those same emissions standards have been subsequently incorporated in the current Industrial Emissions Directive (IED) (2010/75/EU).

Hazardous and medical wastes are handled by other types of incinerators and are not included in this study.

Using modelled emissions allowed us to take account of the volumes of MWI emissions (measured within the flues) and meteorological conditions (wind strength and direction etc.) to give the best scientific estimate of likely exposures from a MWI.   

Even though distance is a cruder measure of exposure with greater potential for exposure misclassification, there were several reasons for also using it. Most early studies have used distance, so including this in our study allowed us to make comparisons. It also may be a proxy for non-flue (non-chimney) related exposures such as road transport to incinerators.

PM₁₀ is shorthand for any particulate matter in the air that has a diameter of 10µm or less (1 µm is a millionth of a metre). Therefore PM₁₀ is small in size and invisible to the naked eye. PM₁₀ particulates can come from both natural (such as dust blown from open land, wildfires etc.) and human activities (such as emissions from motor vehicles, wood burning stoves, dust from construction sites, industrial sources etc.).

All MWIs must comply with Chapter IV of the European Union Industrial Emissions Directive (2010/75/EU) (EU-IED), which is enforced by the Environment Agency (EA) in England, Natural Resources Wales (NRW) in Wales, and the Scottish Environment Protection Agency (SEPA) in Scotland, through Environmental Permitting Regulations.

The EU-IED incorporated the Waste Incineration Directive (2000/76/EC) (EU-WID) that was implemented in Great Britain on the 28th of December 2002 for new incinerators. Existing incinerators had until the 28th of December 2005 to comply. The EU-WID/EU-IED Chapter IV limits Municipal Waste Incinerator (MWI) emissions to a daily average total dust limit value of 10 mg m-3 per flue, as well as providing emission limit values for a number of other pollutants.

There was no clear change in particulate emissions overall following the adoption of EU legislation on MWIs in the UK, which may be due to facilities already meeting the EU standards at the time they were introduced. This was investigated in the papers by Douglas et al. (2017) and Freni-Sterrantino et al. (2019).

We used a standard and well-validated dispersion model (ADMS) to estimate exposures at each postcode near an incinerator. The model uses a series of equations and algorithms to simulate how  pollutants disperse in the atmosphere.

We did additional work to see if plumes from MWI stacks reach the ground by comparing the ratio of PM10 to NOx concentrations in a MWI stack with those found at ground-level air pollution monitoring sites within 10 km of four MWIs (Edmonton, SELCHP, Tyseley and Wolverhampton) (Douglas et al., 2017). Ratios at the monitoring sites were usually those typically seen from traffic sources, suggesting that emissions from MWIs are diluted before reaching ground-level.

The study is funded by Public Health England (PHE), the Scottish government, the Medical Research Council MRC-PHE Centre for Environment and Health and the National Institute for Health Research Health Protection Research Unit (NIHR HPRU) in Health Impact of Environmental Hazards at King’s College London and Imperial College London (HPRU-2012-10141) in partnership with Public Health England (PHE).

The work was conducted at the UK Small Area Health Statistics Unit, which is funded by Public Health England as part of the MRC-PHE Centre for Environment and Health, funded also by the UK Medical Research Council (under grant MR/L01341X/1).

Births and deaths data were from the Office for National Statistics (ONS) National Mortality, Births and Stillbirth registers for England and Wales and the National Health Service (NHS) Numbers for Babies (NN4B). Welsh births data were from the National Child Community Health Dataset (NCCHD) from the NHS Wales' Informatics Service (NWIS)/ Health Solutions Wales (HSW). Scottish births and deaths were from the Information Services Division (ISD) Scotland.

Incinerator emissions data came from the Environment Agency (EA), Scottish Environment  Protection Agency (SEPA), and Natural Resources Wales (NRW).

Data on industrial sites came from the Environment Agency Environmental Permitting Regulations – Industrial sites (England), Natural Resources Wales - Environmental Permitting Regulations –

Industrial sites and the Scottish Pollutant Release Inventory.

2011 Census aggregate data came from the Office for National Statistics (for England and Wales) and the National Records of Scotland (2016): UK Data Service (Edition: June 2016). Road length data came from Meridian 2014 road lengths. Ordnance Survey data © Crown copyright and database right 2014. CACI tobacco expenditure data is © Copyright 1996-2014 CACI Limited. We attest that we have obtained appropriate permissions and paid any required fees for use of copyright-protected materials.

English data on congenital anomalies are from the British and Irish Network of Congenital Anomaly Researchers (BINOCAR) as well as individual regional congenital anomaly registers (RCARs): Congenital Anomaly Register for Oxfordshire, Berkshire and Buckinghamshire (CAROBB); East Midlands & South Yorkshire Congenital Anomaly Register (EMSYCAR); Northern Congenital Abnormality Survey (NorCAS); South West Congenital Anomaly Register (SWCAR); Wessex Antenatally Detected Anomalies Register (WANDA)

Terminations of pregnancy for fetal anomaly (TOPFA) data were from the Department of Health.