Evidence reviews
A series of reviews were conducted to obtain the evidence set out in , with the exception of topic 1(c) which used a modelling approach (see below), and topic 2(b) for which recently developed models combining risk data for multiple combustion sources were the primary source. below shows how evidence has been reviewed or generated through models and how the evidence quality was assessed. Assessment of the overall quality of evidence for each topic is provided in Annexes 4–7, and full details of the rationale and methods used are available in Methods used for evidence assessment available at: http://www.who.int/indoorair/guidelines/hhfc.
As noted in Section 2.2.1, the evidence was summarized in different ways, depending on whether a new systematic review was conducted and reported in full, or whether existing (mostly published) systematic reviews were summarized.
Summaries nitrogen dioxide (of reviews were used where evidence on a range of outcomes was required and space would not have allowed full reporting of all systematic reviews, and/or high quality systematic reviews on the topic had recently been published. Where it was judged important to combine systematic review findings with other evidence, a synthesis was included. In one case (climate impacts and finance) a narrative review was judged to be the best approach, given the complex, multidisciplinary nature of this issue and the fact that this evidence served as context for implementation of the recommendations.
As noted, new systematic reviews were conducted for the purposes of these guidelines, unless a recent completed review meeting content, quality and peer-review criteria was available. In practice, this applied to recently published systematic reviews of (i) risks of asthma and wheeze in children with gas cooking and NO2 exposure (16) and (ii) health risks of kerosene use (17). In each case, the methods (key questions, search terms and strategy) for the published review were assessed, and a summary prepared (see Review 5, Section 4: Health risks of gas; Review 9: Health risks from kerosene).
For all new systematic reviews, search, data extraction and study quality assessment methods (described in and in detail in the full texts of the systematic reviews, see: http://www.who.int/indoorair/guidelines/hhfc) were broadly similar. The details varied according to the type of evidence incorporated, e.g. laboratory testing, epidemiological studies, policy and case studies of adoption. There are also some variations in databases searched (in part as appropriate to the topic) and in languages included. For the systematic reviews of coal use (Review 8), Chinese language studies were included because a high proportion of the world's coal using households are located in China and important research had been conducted there. For other topics, where non-English languages were included (e.g. Chinese) it was found that searching databases in other languages made little, if any, difference to the included set of studies.
The quality of individual studies contributing to these reviews was assessed using standard methods applicable to the type of study. This varied considerably, ranging from laboratory emission studies to epidemiological studies, and case studies of implementation programmes. A summary of the methods used is provided in column 3 of , with further details in the full texts of the reviews (available at: http://www.who.int/indoorair/guidelines/hhfc).
Methods used for assessing the quality of the overall evidence provided by these reviews are described in section 2.3 below, and summarized in column 4 of .
Emissions model
In order to select a model suitable for the purposes of these guidelines, three commonly employed methods were reviewed (see full description in Review 3). Each of these combines the rate of pollutant emission (in terms of mass) within a room (e.g. kitchen) with mathematical models of pollutant transport and fate to provide estimates of indoor pollutant concentrations. These three types of model range from simple constructs to complex computer-based simulations and all have the capacity to provide indoor concentration estimates indicative of those observed in homes due to the device and fuel in question. These are:
The single zone model, which assumes that the pollutant emitted into room air is uniformly mixed throughout the space. Concentration is determined by emission rate and a number of other factors that can be incorporated into the model, including duration of combustion, room volume and air exchange rate.
The three zone model, which divides the room into three zones – a plume rising above the combustion device; warm air within a given distance from the ceiling; and the rest of the room. It is assumed that uniform mixing occurs in each zone. In other respects, this approach is similar to the single zone model.
The computational fluid dynamics model, which considers the forces involved in determining transport of air and pollutants within a room, by dividing the space into a large (or very large) number of small units, and developing equations incorporating momentum, thermal energy and conservation of mass for determining the resulting air pollutant concentrations.
Single zone models have been applied in work on household energy and air pollution for around 30 years, and this approach was adopted for the current purposes. The single zone model has the merit of simplicity in respect of the assumptions used. This is important when developing an approach that can be applied to populations. Such a model needs to account for a wide variation in factors (i.e. room size, air exchange rate, and duration of device use) which determine area concentration for any given emission rate. These have been incorporated by using a range of empirically-derived values for each factor combined in a Monte Carlo simulation. The input data used for the model were obtained from measurements made in India, and are summarized in .
Input distributions for air exchange rates and kitchen volumes.
The output of the model for any given emission rate is therefore a distribution of air pollution concentrations, which can be used to describe the percentage of homes that achieve a specific air pollution goal, such as those in the air AQG. Examples of these distributions are provided in the full description of the model (Review 3). This modelling approach can be applied to any of the pollutants for which AQGs have been determined by WHO, if emissions data are available. For practical reasons, the model has been used to provide guidance just for PM2.5 and CO, as these pollutants together serve as sufficient indicators of the health damaging potential of household fuel combustion in most situations.
Assessment of the quality of this evidence for the purpose of these guidelines (i.e. providing guidance on emission rates that will allow the AQGs to be met), is based on validation studies. The approach to this assessment is summarized in column 4 of , described in more detail in Annex 4 (Assessment of evidence for Recommendation 1), and in full in Review 3.