Small cell lung cancer (SCLC) accounts for ~15% of all lung cancers in the USA (1). The majority of patients typically present with symptoms associated with central airway disease, such as cough and dyspnea, as well as symptoms of widespread metastatic disease, such as weight loss, debility, bone pain and neurologic compromise. Histological analysis may be sufficient for the diagnosis of SCLC, which presents as poorly differentiated tumor that is categorized as a high-grade neuroendocrine carcinoma. The majority of SCLCs are immunoreactive for keratin and thyroid transcription factor-1 (TTF-1). Furthermore, at least one marker of neuroendocrine differentiation, such as neuron-specific enolase (NSE), neural cell adhesion molecule (CD56) or synaptophysin (2), is observed in 75% of SCLC cases (2,3). Despite its rapid doubling time and early development of widespread metastases, SCLC is highly sensitive to chemotherapy and radiotherapy (4). At present, the etoposide and cisplatin regimen is the standard first-line chemotherapy treatment for patients with SCLC. However, the majority of patients exhibit recurrences and subsequently succumb to the disease (5,6). Patients with SCLC are usually classified as either limited-or extensive-stage. Limited-stage SCLC is defined as cancer within only one lung and/or in the lymph nodes in the mediastinum. The majority of patients with SCLC are extensive-stage at the time of initial diagnosis, where the cancer has spread either to the other side of the chest or to more distant locations in the body. The median survival rates are 10.3 months and 5.6 months for patients with limited- and extensive-stage disease, respectively (7).

Smoking is, undoubtedly, the primary cause of lung cancer (8,9). Environmental and occupational exposures are also potential risk factors, but the effects of these suspected factors remain equivocal (10,11). Mosquito coils are a major type of residential insecticide, and are frequently burned indoors and outdoors in Asia. In China, mosquito coils that contain pyrethroid insecticides, particularly d-allethrin, use different concentrations of octachlorodipropyl ether (S-2) as a synergist or an active ingredient (12). S-2 has been reported to exhibit carcinogenicity following its degradation to the extremely potent lung carcinogen, bis(chloromethyl) ether (BCME) (13). The carcinogenicity of BCME was first demonstrated in 1968 with skin painting in mice and subcutaneous injection in rats (14), subsequently a number of additional animal experiments have been performed. Van Duuren et al (14) stated that BCME produced a moderate tumor response as an initiator and caused hyperplasia in the treated area. Preceding the increase in DNA synthesis, an inhibition of DNA synthesis immediately followed treatment with BCME. The reduction in apparent DNA synthesis caused by the agent may occur via a number of mechanisms (15), including: i) Interference with strand separation and/or DNA polymerase activity due to the presence of a covalently bound foreign molecule in DNA, ii) reduction in the number of cells synthesizing DNA due to of cell death or other toxic reactions, iii) breakdown of (labeled) DNA following treatment, and iv) defective enzymes resulting in a failure to utilize nucleoside precursors. Furthermore, BCME is a unique alkylating agent due to the participation of the oxonium ion in addition to the carbonium ion usually encountered with alkylating agent carcinogens. The existence of this equilibrium reaction and the resonance stabilization in the ionic species explains the induction of lung cancer in animals and humans by this carcinogen and its potential for resulting in the induction of malignant tumors at distant sites (16). Exposure to BCME via inhalation has been associated with the formation of lung tumors in rats and mice (17–19). Furthermore, previous studies have shown that BCME leads to mutations in bacteria, as well as unscheduled DNA synthesis in cultured human cells (15,20). In addition, epidemiological studies from various geographical locations, including the USA (21), Germany (22) and Japan (23) have found that occupational exposure to BCME is associated with the development of lung cancer, in particular SCLC (16,20). The present study describes three cases of small cell lung cancer (SCLC) that were likely to have arisen due to exposure to mosquito coils. Consent was obtained from the families of the patients.

In China, the burning of mosquito coils is a common practice, particularly in summer. At present, a limited number of toxicity studies have been published that describe the potential carcinogenic effects associated with exposure to coil-containing toxins. A previous case-control study (24) investigated whether exposure to mosquito coil smoke was a risk factor for the development of lung cancer. The results of the study revealed that mosquito coil smoke exposure was more commonly observed in lung cancer patients than in control subjects (38.1 vs. 17.8%; P<0.01), and that the risk of lung cancer was significantly increased among those who frequently burned mosquito coils compared with non-burners (adjusted odds ratio=3.78; 95% CI, 1.55–6.90) (18). Another study demonstrated a positive association between daily exposure to mosquito coils and lung cancer among smokers in China (25). Therefore, exposure to mosquito coil smoke may have a significant role in the pathogenesis of lung cancer.

The key to recognizing cases of lung cancer that are a result of occupational or environmental exposures is clinical investigation and consideration of all possible causes for the disease that are present. The histological type of occupational-induced lung cancers is usually different to that of lung cancers caused by another factor due to the difference in the etiology of lung cancer. Between 2008 and 2012, three patients were diagnosed with SCLC at the Shanghai Pulmonary Hospital. These patients, whose cases have been reported in the present study, were aged around 40 years old, which is younger than the general age of patients with lung cancer. Notably, all patients had been employed in the manufacture of mosquito coils in Xinghua, Jiangsu, for a mean of 9.1 years. The mean overall survival time after presentation was 10.7 months. The primary routes of occupational exposure to mosquito coil toxins are inhalation and dermal contact. Based upon the clinical and pathological findings, it was hypothesized that occupational exposure to mosquito coils was a major factor involved in the pathogenesis of the lung cancer.

Mosquito coils generally consist of an insecticide, a binder, organic fillers capable of smoldering and additives, such as synergists. It is reported that the majority of mosquito coils used in China contain different concentrations of S-2 (26). Since mosquito coils that contain S-2 are unregistered and illegal for use in the United States (10), global studies regarding the effects of S-2 are limited. Exposure to S-2 by inhalation results in oxidative damage to the liver, spleen and lungs of mice. The lung, however, is believed to be the main target organ (27). Since BCME can be produced from impurities contained in S-2, or by the thermolytic degradation of S-2, the use of S-2-containing mosquito coils has been established to be a potential contributor to environmental exposure to BCME (13). The risk of lung cancer increases with increased exposure duration or cumulative exposure (22). A previous study revealed that for workers exposed to the highest doses, the relative risk of developing lung cancer increased more than 10 fold (28). A further study revealed that the mean time between first exposure and diagnosis was 13 years, and that the average age of exposed individuals at diagnosis was 10.5 years lower than of non-exposed individuals (29).

Histological evaluation has indicated that exposure to S-2 results primarily in small cell-type lung cancers, particularly oat-cell carcinoma (29,30). Based upon the histological evidence, it was hypothesized that the inhalation of S-2 may have been the potential cause of SCLC in the three patients included in the present study. However, the other toxic products released following mosquito coil use have yet to be adequately assessed, therefore, future controlled studies should be conducted in order to evaluate their effects. Exposure is a controllable factor, and workers therefore deserve preventive actions in order to reduce exposure to toxins in the workplace. Furthermore, the effects of daily use and exposure to mosquito coils should be evaluated with respect to further health implications.