The simplest ecological model of health is shown in Figure 1, below. It describes the interaction between the host (i.e. a human being), the environment and a disease (typcially a germ). According to Van Leeuwen16 and Dever27, this model captures the understanding of health that was prevalent in the late 19th Century.
There is a dynamic equilibrium between the three elements. A change in the condition of any one of these three may tip the balance towards either the host or the disease.
Although this model is very simple, it does demonstrate that health is a dynamic equilibrium and it also crudely captures all of the influences on health. More nuanced ecological models do not add further dimensions but instead break down each of the three dimensions here into ever smaller categories. However, this simple model does not come close to describing the relationship between health, disease and environment as it is currently understood. For example, certain agents can cause more than one disease, many diseases have multiple causes, many diseases or health conditions are non-infectious and exposure to a disease vector does not necessarily lead to disease.
Figure 1: A basic ecological model of health
The Ecological Model described above was refined by Morris in 197528. Morris’s version is displayed in Figure 2, below.
Host
Environment
Disease
Figure 2: Morris's socioecological model of health
Morris addressed some of the shortcomings of the original ecological model. He notes that there are two distinct types of environmental factors and replaces the
‘agent’ of disease from the original model with ‘personal behavioural factors’.
The model assumes that behavioural factors have a bigger impact on disease than the physical environment as disease is dependent on where one chooses to live (the choice of residence being the key behavioural factor). It is
questionable, however, how much choice people can exercise over where they live. Many groups in society have no say in where they live: children, the
elderly, those living in institutions and those living in social housing. This model dispenses with the implication that one agent causes one disease to a
multifactorial model for both infectious and non-infectious disease. The model places importance on environmental and behavioural influences on disease although it still has disease (and its absence) as its core concept rather than health.
Shortly after the release of the Ottawa Charter, Hancock published his Mandala of health29.
Genetic
Experiential HOST FACTORS
PERSONAL BEHAVIOURAL FACTORS
EXTERNAL ENVIRONMENTAL FACTORS
Physical
Social
Figure 3: Hancock's mandala of health
Hancock gave this Mandala the subtitle “a health model of the human
ecosystem”. An ecosystem is the collection of components and processes that comprise, and govern the behaviour of, some defined subset of the biosphere.
In this case the subset of the biosphere is human health and Hancock’s Mandala is his attempt to model the systems and processes that govern health. It is a development of Lalonde’s health fields concept and all four fields can be seen in various guises in this model. The influences on health are represented by three concentric circles of nested systems centred on the individual: the family, the community and human-made environment, and finally the culture or biosphere.
Hancock made it clear that the three rings should be interpreted as three dimensional and dynamic; that their relationships with each other would be multi-faceted and would change according to the spatial and temporal context of the individual at the centre. In addition, the author specifies four subgroups of health influences spanning the family and community circles: personal
behaviour, the psycho-socio-economic environment, the physical environment and human biology. Again, the authors intended that these four subgroups should not be viewed as rigid or independent of each other and that their relative importance would vary with place and time.
Hancock’s model was the first to describe a hierarchy of influences on human health16. It also served as the foundation of practice for health policy makers by showing that no single determinant of health should be the exclusive focus of
effort. Instead it encourages multi-level and multi-disciplinary approaches to improving health.
However, Hancock has himself admitted to some weaknesses in this model30. The model fails to address what he views as two key determinants of health.
Firstly the mandala has nothing to say about making the economy or the environment sustainable. Clearly the health of individuals will suffer if the economy is structured in such a way that natural resources such as foodstuffs and fuels are depleted or if the creation of capital pollutes the environment.
Secondly, he pointed out that his model did not make specific reference to the concept of equity of income, citing the work of Wilkinson, who argues that there is an association between income inequality and life expectancy31.
In 1991, Dahlgren and Whitehead32 introduced their hierarchical ‘rainbow’ model of health.
Figure 4: Dahlgren and Whitehead's Model of Health
This model was originally created as a background to a document exploring policies in EU countries that were intended to reduce inequalities in health33. In the accompanying text, Whitehead expounded the idea that inequalities in health resulted from social gradients operating at all levels of her model and also that factors conducive to health (at any level of her model) had less positive influences as social status declined.
Figure 5: The Health Determinants Model
The model of Evans and Stoddart is the most complex that will be discussed here. This is the first model that includes feedback loops such that health and function is not the ‘end product’ of the model but another node in a complex loop of inter-related determinants. For example, if an individual suffers a loss of health and function then it is plausible to say that his level of prosperity will be adversely affected. This might entail him moving to a less salubrious location where the physical environment might encourage the development of further disease (e.g. moving to poor quality housing where problems with damp might exacerbate lung function problems). When an individual suffers disease then his health and function is further compromised. Working in the opposite direction, improved individual prosperity allows greater choice over one’s physical
environment and social environment. Both of these can influence behaviour (for example, diet) and thereby protect and improve the individual’s health.
The model was welcomed for providing insight into the social determinants of population health but Evans and Stoddart received criticism for their belief that prosperity from economic development would lead to health improvements across the whole population. They overlooked the link between advanced economic development and health inequalities34.
These ecological models of the determinants of health are most useful for understanding the different health outcomes in different populations at a given time. They are less good at following outcomes that result from conditions at earlier stages of life and over periods of history.