Hydrogen fluoride damage to vegetation from peri-urban brick kilns in Asia: A growing but unrecognised problem?

Abstract

The rapid urbanisation of many cities in south and south-east Asia has increased the demand for bricks, which are typically supplied from brick kilns in peri-urban areas. We report visible foliar damage to mango, apricot and plum trees in the vicinity of traditional Bull’s Trench brick kilns in Peshawar, Pakistan. Visible injury symptoms, hydrogen fluoride concentrations in air, and foliar fluoride concentrations were all greater in the vicinity of brick kilns than at more distant sites, indicating that fluoride emissions from brick kilns were the main cause of damage. Interviews with local farmers established the significant impact of this damage on their livelihoods. Since poorly regulated brick kilns are often found close to important peri-urban agricultural areas, we suggest that this may be a growing but unrecognised environmental problem in regions of Asia where emission control in brick kilns has not been improved.

Introduction

Hydrogen fluoride (HF) is one of the most phytotoxic air pollutants (Weinstein and Davison, 2003). HF and other fluoride compounds in the atmosphere are deposited to vegetated surfaces either in gaseous form or in the form of particulates. Airborne gaseous fluorides can also enter directly into the leaf through stomata. This fluoride then dissolves in the apoplast, altering the photosynthetic process, causing visible injury and ultimately affecting growth and yield. The impact of atmospheric fluoride pollution from various sources on crops has also been well documented (e.g. Brewer, 1960, Gilbert, 1975, Murray, 1981, Murray, 1983, Mason et al., 1987, Moreas et al., 2002).

Brick kilns, along with aluminium smelters, ceramic manufacture, and phosphorus fertiliser factories, are the major sources responsible for atmospheric fluoride pollution (Weinstein and Davison, 2003). Bricks are produced from soil (usually clay) that may contain fluoride at concentrations up to 500 ppm (Jha et al., 2008) in brick kilns at temperatures ranging from 900 °C to 1150 °C. At these temperatures, fluoride compounds are released into the atmosphere, generally in the form of gaseous HF, silicon fluoride and particulate calcium fluoride, along with other pollutants such as sulphur dioxide (SO₂). The rapid urbanisation of many cities in south and south-east Asia has increased the demand for bricks, which are typically supplied from brick kilns in peri-urban areas (CATF, 2010). Although measures to improve emission control have been introduced in several countries, especially to reduce particulate emissions (CATF, 2010), many of the brick kilns in Asia remain poorly regulated, use low quality fuel, and may contribute significantly to local sulphur and black carbon emissions (Emberson et al., 2003, EPA Pakistan, 2007, Iqbal and Oanh, 2011). Sun and Su (1985) and Lee et al. (2003) reported damage to vegetation around brick and ceramic factories in Taiwan, and local effects on vegetation of fluoride emissions from aluminium factories and thermal power plants have been reported in India (Lal and Ambasht, 1981, Pandey, 1981, Pandey, 1985, Narayan et al., 1994). However, in general, despite the potential high fluoride emissions and the threat to crops, the impact of these brick kilns on agricultural production and farmers’ livelihoods in Asia is poorly understood.

Here we report the results of a case study of these impacts in and around Peshawar city, in north-west Pakistan, which is one of the biggest cities in Pakistan. There are approximately 400–450 brick kilns situated in and around Peshawar city (EPA Pakistan, 2007), and the numbers are increasing rapidly driven by increased construction activity. As a monthly average, it is estimated that a single brick kiln in this region produces about 800,000 bricks, using large amounts of rubber tyres to start the fires, and burns a total of 8 tons of low-quality coal or 20 drums of used vehicle oil (EPA Pakistan, 2007); this leads to significant air pollution emissions. No measurements of fluoride pollution have been carried out to date in and around Peshawar city and the impacts of brick kiln activity on crops in this region are unknown. This research focussed on the effects on local fruit trees, because the area around Peshawar is considered to be the best in Pakistan for apricot and plum production, and both of these species are known to be sensitive to fluoride (Solberg and Adams, 1956, VDI, 1987).

The specific objectives of the study were: (1) to assess the HF concentrations close to brick kilns using passive samplers; (2) to determine the extent of visible injuries typical of HF on fruit trees; (3) to determine whether visible injuries are associated with elevated foliar fluoride concentrations; and (4) to assess, through interviews with local farmers, their awareness of the problem and the severity of its impact. We also consider the wider implications of these results in the context of brick kiln emissions in Asia.