Peak expiratory flow rate and chronic respiratory symptoms among restaurant workers: a cross-sectional study from Thailand

Introduction

Recently cooking fumes have received increased public attention as an indoor and outdoor source of air pollution. The World Health Organization estimated that in 2018 inefficient cooking using solid fuels (biomass, kerosene and coal) caused premature death of about 4 million people worldwide¹. Besides smoke from burning fuel, high temperature treatment of food will generate fumes from the degradation of sugars and fats, as well as from pyrolysis of proteins and amino acids. Previous studies have clearly established that cooking fumes commonly contain fine particulate matter and many other toxic compounds, including volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), aldehydes, alkanoic acids, nitrogen dioxide (NO₂), and carbon monoxide (CO)^2–4. The exact composition and concentration of cooking fumes depend on several factors, including cooking temperature, cooking methods, and cooking oil/fuel type⁵. Other research has reported that the cooking method releasing the most particles, especially in the ultrafine size range, is deep frying, followed by regular frying, stir frying, boiling, and steaming^6,7. Concentrations of volatile organic compounds can vary from 257.5 to 3,494.0 µg/m³, depending on cooking style³. A study of two Chinese cooking styles found concentrations of fine particles, mostly fatty acids, to be 1,406.3 ± 293.4 µg/m³ for Hunan cooking and 672.0 ± 295.8 µg/m³ for Cantonese cooking⁴. It has been estimated that cooking with natural gas could add 21–30% to weekly average indoor concentrations of CO and 25–39% to those of NO_2, depending on ventilation and season of the year⁸.

People working in a kitchen, whether at home or in a restaurant setting, are at risk of exposure to cooking fumes and related health consequences. A study of people using natural gas to cook at home without a ventilation hood predicted that they would be exposed to NO_2, CO, and formaldehyde at 62%, 9%, and 53% above established safety limits, respectively⁸. Lai et al.⁹ reported a close correlation between levels of toxic chemicals in restaurant air and levels of those same chemicals in the urine of chefs. Exposure to cooking fumes can result in various kinds of respiratory problems. A recent study from Taiwan reported an increased risk of lung cancer among chefs cooking Chinese food¹⁰. In places that tend to have modern kitchens, such in Norway, an elevated level of respiratory complaints and chronic bronchitis was also found among professional chefs, especially those who performed a lot of frying¹¹. Studies from various parts of the world have reported an elevated prevalence of both acute and chronic respiratory symptoms and diseases among people involved in cooking^12–18.

Lung function is another effect that has been associated with cooking fumes. A large study of elementary school children found reductions of peak expiratory flow up to 3.4% among children in families cooking with gas¹⁹. A study of Chinese restaurants in Hong Kong reported that workers cooking with gas had poorer lung function than those cooking with electricity¹³. A similar result was also observed in a study from India². In Nigeria, workers exposed to wood smoke and oil fumes from a local style of grilled meat called “mai suya” have lower forced expiratory volume in one second (FEV1) and lower forced vital capacity (FVC) compared to an unexposed control group¹⁸.

In Thailand, four common types of eateries are: ‘tamsang’ restaurants (a kind of Thai restaurant that makes a variety of foods to order, from the Thai word ร้านอาหารตามสั่ง), noodle restaurants, papaya salad restaurants, and roadside barbecue stalls²⁰ The kinds of food available at these four sorts of eateries are not the same. Of the four, ‘tamsang’ restaurants sell the widest variety of food, mainly popular Thai dishes, including red curry, basil stir fry, fried rice and numerous other fried dishes. Most of the food from ‘tamsang’ restaurants is cooked at high temperatures, creating a lot of oily steam and pungent vapors from frying chili peppers and other spicy ingredients. In noodle restaurants, much of the food is boiled but some noodle restaurants also make fried dishes, like phat Thai and other fried noodle dishes. At roadside barbecue stalls, various kinds of meat (beef, pork, chicken, and seafood) are grilled, usually over charcoal, producing abundant smoky vapors. In papaya salad restaurants, the salad is often served with some side dishes, such as sour soup, spicy meat salad, and grilled chicken. Therefore, papaya salad restaurants do employ a variety of cooking methods, including boiling, frying, grilling, but there is relatively less frying than at a ‘tamsang’ restaurant and less grilling than at a barbecue stall²⁰.

To date, there has been no epidemiological study to show the variation in health effects resulting from work at different kinds of restaurants in Thailand. A previous study did find a higher occurrence of respiratory symptoms among restaurant workers in general, but due to small sample size, the study was not able to reveal a statistically significant difference in health effects between different types of restaurants¹⁷. The current study investigates peak expiratory flow rate (PEFR) and chronic respiratory symptoms among workers at different types of restaurants in Thailand. The study results are useful for the prevention and control of occupational health problems among restaurant workers. The purpose of this study is to investigate if cooks working in different types of kitchens have different rates of impaired lung function and occurrence of respiratory symptoms.

Methods

This study is a cross-sectional study.

Results

Most of the study participants were female cooks with a similar mean age across all four types of restaurants studied. Most of the participants had been working for more than one year. Less than 20% of the participants were current cigarette smokers. Additional information on the demographic data is shown in Table 1 and underlying data²⁵.

It was found that restaurant workers had a high variation in lung function performance with the lowest average peak expiratory flow rate (PEFR) of 278 ± 113 among those working in ‘tamsang’ restaurants and the highest PEFR of 356 ± 107 among workers in noodle restaurants (Table 2). Compared to standard PEFR values of Thai people, 41.2%–64.7% of restaurant workers had a poor PEFR, defined as PEFR (measured)/PEFR (standard) less than 80%, and the prevalence of poor PEFR varied significantly across the types of restaurants (p<0.001). Many of the workers reported having chronic cough, phlegm, wheezing, moderate dyspnea, and severe dyspnea, but only moderate dyspnea showed a variation among types of restaurants.

Further analysis using multiple logistic regression showed that those who worked in ‘tamsang’ restaurants are at a significantly higher risk (p <0.05) of having poor PEFR (OR = 2.59, 95% CI 1.33–5.06), and moderate dyspnea (OR = 3.79, 95% CI 1.63–8.79) compared to papaya salad restaurant workers (Table 3). A higher risk of poor PEFR and/ or chronic respiratory symptoms were also significantly associated (p <0.05) with the use of palm oil for cooking, with having frequent TWC, use of a ventilation hood, and with working in 1–6 m² kitchen. All analyses were carried out using the following covariates: age, gender, BMI, cigarette smoking, and cooking at home.

Discussion

This study revealed that a large proportion of restaurant workers have poor PEFR, with the highest frequency of 64.7% found in ‘tamsang’ restaurants and the lowest rate of 41.2% in papaya salad restaurants (Table 2). Compared to workers in papaya salad restaurants, workers in ‘tamsang’ restaurants are more likely to have a low PEFR (OR = 2.59, 95% CI 1.33–5.06) and moderate dyspnea symptoms (OR = 3.79, 95% CI 1.63–8.79) (Table 3). These results are consistent with previous literature, which has reported that cooking fumes contains pollutants, such as fine particulate matter, acrolein, formaldehyde, and NO₂, that can cause airway irritation^2,26,27. Although no relevant data from Thailand appears to exist, it is expected that Thai style cooking might generate more of these pollutants than other literature has reported in some other styles of cooking, because Thai cooking more often involves stir frying at high temperature, which is similar to Chinese cooking, but Thai cooking often uses more spices and other pungent ingredients. These pollutants are found more often in high temperature frying than in boiling or steaming^3,28, which poses a real hazard to workers at ‘tamsang’ restaurants, since, as mentioned earlier, ‘tamsang’ restaurants do a lot of frying. More pollutants can also be expected from barbecue stalls, which usually grill meat with charcoal. However, this type of eatery is usually located outdoors in the open air, therefore exposure can be greatly reduced by the wind. Also somewhat surprising in this study, ‘tamsang’ restaurant workers were found to have a lower risk of severe dyspnea (OR = 0.45, 95% CI 0.20–0.99). This might be the result of the so called ‘healthy worker effect’ (HWE), in which people who have severely illness are excluded from the job²⁹. Similar findings were also observed in the association between working as a cook and prevalence of severe dyspnea (OR = 0.33, 95% CI 0.11–0.98) (Table 3). This bias is likely limited to the more severe symptoms like severe dyspnea rather than other symptoms with milder health effects.

This study surprisingly found that compared to cooking with lard, cooking with palm oil is associated with an elevated risk of poor PEFR (OR = 5.38, 95% CI 1.43–20.31) but soybean oil was not (Table 3). This finding seems to contradict information that palm oil and lard contain mostly saturated and monounsaturated fats, and that both are similarly resistant to oxidation and have a high boiling point^28,30. Compared with soybean oil which has a lower boiling point, lard and palm oil would be expected to produce fewer harmful pollutants when cooked. However, a study by Lee et al.³¹ also found a negative effect from palm olein in the form of higher levels of aldehyde when frying chicken with palm olein compared to other cooking oils, despite similar amounts of fatty acids. At present, data on health effects of palm oil is limited. Further research is therefore needed to investigate this issue.

The current study also found that low PEFR is correlated with tears while cooking (TWC), which refers to tearing eyes caused by smoke exposure, in a dose-response fashion, with an OR of 3.07 (95% CI 1.09–8.63) for those who often had TWC and an OR of 2.80 (95% CI 1.29–6.06) for those who only occasionally had TWC (Table 3). TWC was also significantly correlated with many chronic symptoms, including cough, wheezing, and moderate dyspnea. This result confirms that TWC is a good marker for cooking fumes. In a previous study of the current author, TWC was also found to be correlated with respiratory symptoms¹⁷.

This study also showed that not using a ventilation hood is significantly associated with an increased risk of chronic cough (OR = 2.25, 95% CI 1.03–4.94) and severe dyspnea (OR = 2.58, 95% CI 1.14–5.86) (Table 3). This finding was consistent with general thinking and previous research that has found that ventilation hoods, even when operating inefficiently, can significantly reduce cooking fumes exposure^8,32. On the other hand, regarding kitchen location, there were no significant associations found. A study in Ghana similarly found that concentrations of PM2.5 and black carbon did not significantly differ across outdoor, enclosed, and semi-enclosed kitchens⁵.

Inside ‘tamsang’ restaurants, people working in a smaller kitchen area of 1–6 m² have an increased risk of abnormal PEFR (OR = 2.22; 95% CI 1.13–4.36), compared to those working in a larger kitchen area of ≥7 m² (Table 3). One possible explanation is that restaurants with a larger kitchen area tend to have and use ventilation hoods more often than restaurants with a smaller kitchen area. Another possible explanation is that because a large kitchen area has more space, there is a greater volume of air to dilute the pollutants. Further analysis revealed that, in fact, large kitchens use ventilations hoods less frequently than small kitchens (27.5% vs. 44.3%, respectively). Thus, the first idea can be rejected.

Conclusion

Working in a Thai restaurant increases risk of abnormal lung function and chronic respiratory symptoms. Compared to working in a papaya salad restaurant, working in a ‘tamsang’ restaurant is associated with a lower PEFR, and higher rates of moderate dyspnea. Use of palm oil, having frequent tears while cooking, not using a ventilation hood, and working in a small kitchen area are significant predictors of poor lung function and/ or chronic respiratory symptoms. These findings are relevant for anyone concerned with the health and welfare of restaurant workers. Relevant regulatory organizations need to address these dangers in order to protect the health of restaurant workers, particularly those in ‘tamsang’ restaurants.

Data availability