Module 11: Smoke Exposure and Cataracts

Indoor air pollution takes many forms, ranging from smoke emitted from solid fuel combustion during cooking to complex mixtures of chemicals present in modern buildings.  Health risks from indoor air pollution are likely to be greatest in developing countries.  In many households, everyday exposure to air pollution may contribute to an increasing prevalence of asthma, cancer, and, as discussed in this article, cataract. (1)

Risk Factors

Cataracts are the leading cause of blindness and visual impairment worldwide.  In fact, cataract-related blindness accounts for half of the two million people who become blind each year. (2),(3) The prevalence of cataracts is higher in females than in males in developed and developing countries,(4) and in developing countries cataract occurs at an earlier age.(5) Although effective treatment options are available to restore vision, identifying risk factors helps to establish preventive measures.  

In fact, "increasing age is the most important risk factor for cataract, possibly because of the accumulation of lens damage with age together with an age-related increase in oxidative damage in the eye.”(6) Many epidemiological studies have established other risk factors for cataract formation, including:

Recently, it has been shown that exposure to smoke can contribute to the development of cataracts later in life.  This section reviews research on the association between smoke and cataracts, with a specific focus on cigarette smoke and smoke from cooking.

Cigarette Smoke

It is estimated that one third of all women and two thirds of men in India use tobacco in some form, such as smoking tobacco in cigarettes.  This statistic is cause for concern as smoking is reported to be a risk factor for eye diseases such as cataract, age-related macular degeneration, and glaucoma.(13)   

Studies show that cigarettes contribute to the formation of cataracts in two ways.  First, free radicals present in tobacco smoke assault the eye directly, potentially damaging lens proteins and the fiber cell membrane in the lens. (14),(15)  Second, smoking reduces the body's levels of antioxidants and certain enzymes which may help remove damaged protein from the lens. (16),(17) Over time, this damage from smoking can double or triple an individual's risk of developing cataracts.(18) 

Cooking Smoke

In poverty-stricken communities, inadequate housing ventilation and improper cooking stoves pose a danger to women who do most of the food preparation.  Dangerous smoke released from burning unclean solid fuel sources often has no direct path out of the house due to unflued stoves.  One study found that less than 20% of homes in poor areas of northeastern Brazil and central Mexico were safe to live and breathe in. This is because wood smoke contains many chemical products, including carcinogens, carbon monoxide, and hydrocarbons that are detrimental to human health.(19)

Many people in developing countries are forced to use such fuels for cooking because of deforestation, population expansion, and degradation of agriculture and land.  Sources indicate that "nearly half the world's population prepares meals with wood or wood-replacement fuels on primitive stoves without chimneys.”(20)

The consequences of indoor air pollution are revealed in a study conducted in Nepal and India that examined the association between cooking with unflued indoor stoves and development of cataracts.  This study found that the use of solid fuel in unflued indoor stoves is associated with an increased risk of cataract development in women.(21) After controlling for differences in age, educational status, housing structure, type of lighting in the home, and occupation, it was found that the use of an unflued solid-fuel stove is an independent risk factor for the development of cataract.   

Solutions

In order to address the dangers posed by exposure to smoke, several solutions have been suggested.  First, replacing unflued stoves with flued stoves would greatly improve ventilation.  Cooking in an unventilated kitchen doubles the risk of developing cataracts compared with cooking in a fully or partially ventilated kitchen.  By allowing smoke to exit the house, this risk is reduced.

“Worldwide, millions of people, particularly women, are exposed to high levels of indoor smoke from cooking with solid fuels on unvented stoves. The public health benefits of widespread stove improvement, particularly the addition of flues, could be immense.”(22)

However, cooking with cleaner burning fuels would be the best option. Increased ventilation in the kitchen is helpful, but ventilation alone does not provide adequate protection against the effects of solid-fuel stoves.  Thus, the ideal solution would be to replace the solid-fuel stoves with stoves that use liquid fuel or gas.  Since resources are limited in developing countries, vented solid-fuel stoves may be a more economical solution.

“Unfortunately, while the health problems are all too clear, the solutions are as many as grains of sand in the desert. The issues involved are culturally diverse since they relate to such basic traditional patterns as how people live and cook and eat. The literature abounds with designs for 'simple' smokeless stoves or 'elementary' chimneys, hoods and smoke removal appliances. But persuading people to build, install, maintain and use such devices en masse is a thorny issue; indeed the first step is probably to persuade the millions exposed to biomass smoke that it does actually pose health hazard. . ." (23)

In regard to the risks posed by cigarette smoke, cessation is the best way to reduce the chance of developing cataracts.  The benefits of quitting are far-reaching as smoking is a major modifiable risk factor for many chronic diseases.

Twelve studies have assessed the risk of ex-smokers developing cataracts.(24) Ex-smokers have a reduced risk for developing cataracts compared with current smokers, yet their risk is greater than that of individuals who have never smoked.  It has also been found that the greater the intensity of previous smoking, the longer it takes for the increased risk to decline.  Such research suggests that while smoking cessation reduces the risk of cataract development, the effect of cessation takes time and may only be partial.  Education and public health campaigns aimed at increasing awareness of the dangers of smoking may help to curb the prevalence of cataracts in the developing world.

Footnotes

(1) Bruce N, Perez-Padilla R, Albalak R. Indoor air pollution in developing countries: a major environmental and public health challenge. Bulletin of theWorld Health Organization 2000; 78:1078–1092.

(2) West S, Sommer A. Prevention of blindness and priorities for the future. Bull World Health Organ 2001;79:244 48.

(3) Javitt JC, Wang F, West SK. Blindness due to cataract: epidemiology and prevention. Annu Rev Public Health 1996;17:159–77.

(4) Lewallen S, Courtright P. Gender and use of cataract surgical services in developing countries. Bull World Health Organ 2002;80:300–03.

(5) Javitt JC, Wang F, West SK. Blindness due to cataract: epidemiology and prevention. Annu Rev Public Health 1996;17:159–77.

(6) Kelly SP, Thornton J, Edwards R, et al. Smoking and cataract: review of causal association. J Cataract Refract Surg 2005;31:2395–404

(7) Hiller R, Giacometti L, Yuen K. Sunlight and cataract: an epidemiologic investigation. Am J Epidemiol 1977;105:450–59.

(8) McCarty CA, Mukesh BN, Fu CL, Taylor HR. The epidemiology of cataract in Australia. Am J Opththalmol 1999;128:446–65.

(9) Bhatnagar R, West KP, Jr, Vitale S, Sommer A, Joshi S, Venkataswamy G. Risk of cataract and history of severe diarrheal disease in southern India. Arch Ophthalmol 1991;109:696–99.

(10) Harding JJ, Rixon KC. Carbamylation of lens proteins: a possible factor in cataractogenesis in some tropical countries. Exp Eye Res 1980;31:567–71.

(11) Foster PJ, Wong TY, Machin D, Johnson GJ, Seah SK. Risk factors for nuclear, cortical and posterior subcapsular cataracts in the Chinese population of Singapore: the Tanjong Pagar Survey. Br J Ophthalmol 2003;87:1112–20.

(12) Mohan M, Sperduto RD, Angra SK et al. India-US case-control study of age-related cataracts. Arch Ophthalmol 1989;107:670–76.

(13) World Health Organization. Tobacco or health, a global status report. Geneva: WHO, 1997.

(14) McCarty CA, Nanjan MB, Taylor HR. Attributable risk estimates for cataract to prioritize medical and public health action. Invest Ophthalmol Vis Sci 2000;41:3720–25.

(15) Van Heyningen R, Pirie A. Naphthalene cataract in pigmented and albino rabbits. Exp Eye Res 1976;22:393–94.

(16) Shalini VK, Luthra M, Srinivas L et al. Oxidative damage to the eye lens caused by cigarette smoke and fuel smoke condensates. Indian J Biochem Biophys 1994;31:261–66.

(17) Wegener A, Kaegler M, Stinn W. Frequency and nature of spontaneous age-related eye lesions observed in a 2-year inhalation toxicity study in rats. Ophthalmic Res 2002;34:281–87.

(18) Kelly SP, Thornton J, Edwards R, et al. Smoking and cataract: review of causal association. J Cataract Refract Surg 2005;31:2395–404.

(20) Ibid.

(21) Pokhrel AK, Smith KR, Khalakdina A, Deuja A, Bates MN. Case-control study of indoor cooking smoke exposure and cataract in Nepal and India. Int J Epidemiol. 2005;34:702–708.

(22) Ibid.

(24) Kelly SP, Thornton J, Edwards R, et al. Smoking and cataract: review of causal association. J Cataract Refract Surg 2005;31:2395–404.