Relationship between psychological distress, mental disorders and smoking
The K6 scale is a useful short measure for assessing mental health problems in omnibus surveys. It was specifically designed for surveys such as the NHIS which seek to assess a large number of topics, and thus need effective, short screening scales. Our results showing that the K6 score was strongly associated with ICD-10 diagnosis of anxiety or depressive disorders are consistent with previous findings [29, 32]. Although the K6 questions ask about the 30 days prior to the survey, we found that most people with high levels of non-specific psychological distress have long-standing mental disorders. This suggests that high K6 scores are unlikely to be identifying short-term or transient mental health conditions. As far as we know, these are the first data showing that high levels of non-specific psychological distress are associated with mental disorders of long duration.
The K6 has an additional advantage of being a dimensional scale, with higher scores being associated with higher levels of psychological distress. It has been suggested that research based on applying ICD-10 or DSM-IV diagnostic criteria may underestimate the contribution of mental illness to smoking behaviours by ignoring the contribution of sub-threshold disorders . Studies that use ICD-10 or DSM-IV diagnoses via structured interviews have estimated that one third of adult smokers in the US have mental disorders [10, 11]. Our results show an increasing trend of smoking rates and decreasing rates of smoking cessation even for moderate levels of psychological distress, consistent with the idea that mental disorders of mild and moderate severity are significant risk factors for smoking and inhibitors of smoking cessation . Thus the contribution of mental illness to smoking initiation, delayed smoking cessation and longer duration of smoking may be substantial.
The relationship between smoking and severe mental illness is well known [39, 40]. Previous research using the K6 scale [41–43] has used the highest band of the scale, scores of 13 and above, to define the category of serious psychological distress, which is highly correlated with severe mental disorder requiring clinical care [28, 29]. Approximately 3% of the US population falls in this range, according to the NHIS data. Studies using structured diagnostic instruments, such as the CIDI, estimate a prevalence of 12-month mental disorders of around 20% in the US and Australia [32, 44]. These studies include more common disorders such as anxiety and depressive disorders that are less severe than the definition of severe mental illness or serious psychological distress. Nevertheless, smoking rates have been found to be high using this broader definition of mental disorders, and even people with anxiety or depressive disorders of mild or moderate severity have higher rates of smoking initiation and lower rates of smoking cessation [10, 11, 13]. Some have argued that the relatively high prevalence rates of mental disorders estimated in epidemiological surveys are unrealistic as specialist clinical services could not be provided to 20% of the population, and that the definition of mental disorders should be limited to those disorders severe enough to require clinical intervention or justify receipt of disability benefits [12, 45, 46]. Physical health problems as a broad group are also highly prevalent, and common chronic conditions such as asthma, hypertension, diabetes and metabolic syndrome are burdensome and require management even if they do not require immediate hospitalisation. Similarly, the broader definition of mental disorders encompassed by ICD-10 and DSM-IV includes disorders that may not require immediate clinical treatment, but are nevertheless burdensome and impact on people's functioning and their ability to contribute to society. Smoking rates in this group are double that of people with no lifetime mental disorders [10, 11]. Our results show that the approximately 20% of the US adult population who score in the 3-7 range on the K6, well below the 13 and above cut-off for defining serious psychological distress, were 40% more likely to be current smokers, were more likely to want to or to try to quit smoking, but were 20% less likely to successfully quit smoking than people with K6 scores in the low range 0-2. While some attention has been directed to addressing smoking among people with severe mental illness in contact with specialist services , one implication of these findings is the need for non-service based responses to address the larger population of smokers with common mental disorders.
Psychological distress and smoking cessation
We found that people with moderate and high levels of non-specific psychological distress were more likely than people with low levels of psychological distress to want to quit smoking and to try to quit smoking. People with moderate and high levels of psychological distress were also just as likely as those with low levels of non-specific psychological distress to use smoking cessation aids. However, they were significantly less likely to have successfully quit smoking for six months or more. Mental illness has been associated with higher levels of nicotine dependence, and higher intensity of smoking [48, 49]. It is possible that this could contribute to lower rates of smoking cessation in people with mental illness.
People with moderate and high levels of non-specific psychological distress may require a higher level of support, or more specifically targeted strategies to support their smoking cessation efforts. Analysis of tobacco industry marketing research and market segmentation studies has shown that historically the tobacco industry has used psychosocial and other personality traits to develop targeted promotional strategies that encourage uptake of tobacco use [10, 50, 51]. A number of different brand variants have been brought to market with specifically targeted promotional strategies that took advantage of industry knowledge of the association between smoking and psychosocial problems [50–54]. In addition to developing brands or campaigns targeting youth or women, for example, the tobacco industry targeted market segments defined by psychosocial characteristics such as personality type. Campaigns and promotional strategies were developed around the use of tobacco products to address psychosocial issues such as reducing stress, anxiety or nervousness, improving mood, increasing confidence in social situations, or improving concentration [51, 54]. In contrast, population-based strategies in tobacco control have much less often been targeted at specific demographics and other population groups . The result is an inequality in outcomes from these population-based investments which seem to disadvantage smokers who are also dealing with mental health problems .
Implications for health promotion and tobacco control
It is well recognised that the profile of adult smokers now is demographically and psychosocially very different from the profile of those who smoked a couple of decades ago . That such a high proportion of current smokers in developed countries suffer from some form of mental illness or psychological distress has been recognised as representing a new and emerging challenge for tobacco control, as these smokers may perceive different benefits to smoking, and efforts to quit smoking may be complicated by their life circumstances and the interaction between psychiatric symptoms and neuroactive substances such as nicotine . Nonetheless, outside of efforts to reduce smoking in psychiatric inpatient settings, policy documents describing directions in tobacco control do not propose any specific strategies for responding to these challenges or providing support to this large and vulnerable group [14, 15, 55].
It is known that many common mental disorders, in particular anxiety disorders, have onset during childhood and adolescence, and persist for many years . Most adult smokers begin smoking and proceed to daily smoking in their teenage years . While the causal pathways are not clearly understood and there is some evidence to support multiple pathways, the strong association between mental illness and smoking, and the common long-term persistence of both smoking and mental illness, suggest the value in responding to both problems in a co-ordinated way [57, 58]. As a large proportion of smokers have common mental disorders, such efforts would likely be helpful to many people. There is some evidence that smoking cessation not only benefits long-term physical health, but may result in improvements in mental health as well . In the short term, smoking cessation can result in exacerbation of anxiety or depressive symptoms, but in the longer term, smoking cessation is associated with an overall reduction in anxiety and stress .
Only a small proportion of people with high levels of non-specific psychological distress, and common mental disorders such as anxiety and depression, are treated by specialist mental health services . The majority of people with these problems do not seek any professional help. Service-based responses to smoking and mental illness, such as smoke-free mental health units, and smoking cessation counselling as part of mental health treatments, will only reach a small proportion of the population of adult smokers with mental health problems. This suggests that targeted population-based strategies will be required. While the treatment of individuals with mental health problems has been moving to embrace a more holistic strategy of treating the whole person including any physical health problems and substance dependence issues they might have , population health has been slower to respond to this trend. Most population health initiatives are targeted at single diseases or risk factors, and are often coordinated by organisations that are focused on single issues or diseases. For instance, organisations that focus on heart disease, or cancer, which traditionally have had a strong interest in population-based tobacco control initiatives have been more likely to regard people with mental illness as the target group of some other organisation rather than a numerically significant, and vulnerable, part of their own target groups.
Changes in the socio-demographic profile of smokers over time, and the persistence of smoking despite long-term advertising of the health dangers have led to the development of the hardening hypothesis and identification of possible hard-core smokers . This hypothesis is controversial , and is supported by only limited empirical data. It is underpinned by the concept of the hard-core smoker as someone who is stubbornly resistant to population tobacco-control initiatives. It is possible that some individuals who continue to smoke despite decades of publicity about the dangers of smoking, are facing other problems in their lives, such as mental health problems, that affect their ability to quit smoking. Better understanding of the characteristics and life circumstances of these heavily dependent smokers may help refocus tobacco control efforts in ways that help address the range of difficulties these people are facing.
Cross-sectional studies, such as the NHIS, describe associations but cannot inform causal pathways. It is not possible to conclude from these data whether psychological distress leads to smoking uptake or whether smoking causes psychological distress or whether both are related to some other causal factors. However, the data from the Australian NSMHWB showed that the vast majority of people who have moderate or high levels of non-specific psychological distress have mental disorders of long duration. It is reasonable to assume that a high proportion of people with moderate and high levels of non-specific psychological distress in the NHIS sample would have had long-standing elevated levels of psychological distress, predating quit attempts in the 12 months prior to the survey, in most cases by many years. A longitudinal study of Swedish male military conscripts found that lower levels of smoking cessation between ages 30 and 50 were predicted by higher levels of psychological distress measured at 18 years .
While the K6 scale has been shown to be a good predictor of mental disorders, and people with high K6 scores are likely to have mental disorders of long duration, K6 scores are not the same as making formal psychiatric diagnoses. Studies that use the K6 and the CIDI are primarily designed to assess mental disorders during discrete time intervals. It has also been suggested that the strong association between anxiety, depression and smoking could be conceptualised in terms of neuroticism - the general personality trait that encompasses long-term and persistent susceptibility to anxiety and depressive symptoms [62, 63]. There is some evidence that short-term mental disorders in the absence of neurotic traits may have a weaker association with smoking behaviours [64, 65]. While the K6 measure of psychological distress was found to be strongly associated with smoking status and inversely associated with successful smoking cessation, a more direct measure of long-standing mental disorders such as anxiety or depression may show even stronger associations.