WHO model for measuring emissions incorrect; may need to revise guidelines for cookstoves: Study
The World Health Organisation (WHO) may need to revise its cookstove-based guidelines for emissions from biomass fuels as the existing model for measuring emissions can overestimate the same, according to a new study by researchers from Johns Hopkins University (JHU), USA.
This means that the Bureau of Indian Standards (BIS) may also need to revise its methodology to estimate the emission rates necessary for standardising improved cookstoves.
In a recent paper, researchers at Bloomberg School of Public Health, department of environmental health and engineering, JHU, estimated the air exchange rates determined from real-time measurements of PM2.5 concentrations in kitchens and living room areas in residences in rural Bangladesh. The findings, which are also relevant to rural India, have been published in the December 2021 volume of the peer-reviewed journal Building and Environment.
Household air pollution impacts approximately 41% of the global population, according to multiple researchers. WHO estimates that 4.3 million people die every year from exposure to indoor air pollution, with 1.7 million of these deaths occurring in the South-East Asian region. Emissions from cookstoves are significant contributors to indoor air pollution.
Fine particulate matter, known as PM 2.5, is an air pollutant suspended in the air that is released from burning of biomass fuels. When present in high levels in the air, PM2.5 can be very harmful to human health.
“The air exchange rate (AER), or the rate at which outdoor air replaces indoor air is a critical parameter for evaluating the quality of ventilation in an environment,” said Darpan Das, a post-doctoral fellow at JHU and the first author of the paper titled ‘Estimating residential air exchange rates in rural Bangladesh using a near field-far field model’.
In 2014, WHO issued guidelines had recommended an annual average indoor air quality concentration of below 35 μg/m, based on air exchange rates. For decades, the measurement for estimates of air exchange rates from PM2.5 emissions and concentrations have been done on the assumption of a one-compartment model.
Such a model assumes that the indoor environment is a homogenous, instantaneously mixed zone. This model is referred to as the well-mixed room (WMR) model of a room of a certain volume, through which the ventilation airflow rate is constant. The air entering the room has a contaminant concentration. There is also a contaminant source within the room that is generating the particulate contaminant at a constant rate.
“The WMR model as the name suggests assumes a uniform concentration throughout the room. It gives a reasonable estimate of exposure intensity for individuals who are not positioned close to the emission source. However, spatial monitoring of kitchen indoor environments air indicates that concentration is higher near the cookstove source than at distant points in the kitchen or other rooms. While it is common to use a WMR model for estimation of ACH in rural households, this assumption is only valid for conditions in rooms that are far away from the cookstove source (e.g., living rooms) and not necessarily for the kitchen, especially very close to the cookstove,” said Das, who has now joined as a scientist at WHO collaborating centre for occupational health located in Edinburgh, Scotland.
In this paper, the researcher shows that the situation is better represented by a two-compartment model. This change results in dramatically lower estimates of the air exchange rates through the residence, and these lower values, in turn, result in very different values for the required emission rates of fine particles from biomass-based cookstoves.
A one-compartment model or well-mixed room model is where the indoor environment is a homogeneous single mixed zone. As opposed to the single zone model with a WMR assumption, the two-compartment model used by the researchers in the study conceptually divides the indoor space into two zones — kitchen and living room.
“The air exchange rate estimated using a one-compartment model is unrealistically high and >20, whereas a two-compartment model yields more realistic values <1. The findings of the present study indicate that the two-compartment model is a better representation of concentrations of pollutants emitted by cookstoves near the source and leads to better estimates of the air exchange rates than the one-compartment model,” Das added. The project was funded by the National Institute for Environmental Health Sciences and Johns Hopkins National Institute for Occupational Safety and Health Education and Research Centre grant.
Das said, “This paper will have potentially significant policy implications for indoor air quality in developing nations. While the one-box compartmental model is appropriate for estimating exposures farther away from emission sources, a 2-box model may be more appropriate for distinguishing between exposures of the cook versus other occupants in the house, as well as estimating ventilation rates in the house.” The revised methodology can be used by WHO to devise the emission rate targets for cookstoves
Ankit Gupta, senior scientist, National Environmental Engineering Research Institute, Mumbai, who was not a part of the study, said, “The AER in rural kitchens is one of the most significant parameters which is less researched while we discuss the dose-exposure of individuals. The individuals’ exposure largely depends upon AER, which is being used to arrive at disability-adjusted life years (DALYs) and health burdens in developing countries. It is the right time that we generate the right data sets resulting in an appropriate model for effective decision making on rural kitchen interventions.”
Srinidhi Balasubramanian, former postdoctoral research associate, University of Minnesota and an incoming assistant professor at the Indian Institute of Technology Bombay, who was not a part of the study, said, “Reducing the health burden from household air pollution is one of the biggest environmental challenges today. We need improved methods to quantify the emissions and impacts of pollutants from indoor sources such as cookstoves. This study presents a way forward, through the use of a two-compartment model instead of a one-compartment model to better model air exchange rates in different microenvironments such as the near-to-source kitchen and the far vicinity living spaces. Such results will definitely be valuable in developing guidelines for low emissions cookstoves. I also see value in using the methodology towards studying indoor air quality in spaces such as hostels, schools, and perhaps even to study the airborne transmission of diseases.”
How emissions are currently measured:
One compartment model or well-mixed room model is where the indoor environment is a homogeneous single mixed zone. Most studies have estimated air exchange rates using this model in rural households.
What this study uses:
The two-compartment model is also known as the near field far-field model (NF:FF). As opposed to the single zone model with a WMR assumption, the NF-FF conceptually divides the indoor space into two zones — kitchen and living room.