Occupational health deals with the principles of public health in workplace settings; it is the study of the exposures and health outcomes in workplaces to ensure that the work has to be fitted for the workers and the workers are fitted for the jobs, and that, the risk to health due to occupations are minimised or prevented \cite{guidotti_GlobalOccupationalHealth_2011} . A broad range of disciplines contribute to Environmental health, such as occupational epidemiology, industrial hygiene, occupational medicine, and others. The ILO/WHO definition is as follows:
Occupational health should aim at the promotion and maintenance of the highest degree of physical, mental and social well-being of workers in all occupations; the prevention amongst workers of departures from health caused by their working conditions; the protection of workers in their employment from risks resulting from factors adverse to health; the placing and maintenance of the worker in an occupational environment adapted to his physiological and psychological capabilities and; to summarize: the adaptation of work to man and of each man to his job. 

Concept 1: protect-adapt-promote-care

Protect workers from illnesses and prevent illnesses among workers. -- This is the core business of occupational health and consists of four elements:
(1) we must be able to identify hazardous exposure,  we must be able to tell when a worker has work overload or adverse work conditions that will potentially lead to poor health;
(2) in the shop floor, we must be able to assess what the worker is exposed to and measure the level of exposure ;
(3) we must be able to assess risks to health of the worker as a result of exposure;
(4) we must be able to prevent illnesses and establish control measures so that we can ensure the worker's health and safety
Adapt the work to the worker and the worker to the work. -- We  must acknowledge that different workers based on their age, gender, mental capacity, physical capacity, health histories are best suited to different types of jobs; similarly, different types of jobs demand different types of working ability. We  must be able to determine what these are and enable people to adapt to those situations. 
Promote health and well-being of all workers. --We must study how people work best, and how we can make proper adjustment of work environment.
Arrange for curative and rehabilitative services when they are available, and we must be able to rehabilitate workers when needed. --- While primary care or primary prevention is the best way to ensure workers will enjoy good health, often prevention and promotion of health is not enough. People often work in scaffolds and have protective helmets but even then, they may fall over and injure themselves. Occupational health specialists should then arrange to have the worker assessed as soon as possible and arrange for his or her rehabilitation if needed.
Establish primary care services. -- Primary care services mean that medical care where a doctor or nurse will be available in the workplace to diagnose and provide medical treatment in the first instance. For example, if a worker feels dizzy, a medical doctor must be available to diagnose the cause of the dizziness: is it because of some exposure in the workplace, is it because the person has problems with his or her spine, or is it because the person may have an aura of migraine? What needs to be done? 

Concept 2: environmental health is different from occupational health

We use the principles of environmental health to occupational settings as we can argue that occupations or workplaces are specific environments. So the basic principles that in working environment too, workers or objects that change the conditions of the environment affect our health applies. We need to understand that there are exposures to which workers are exposed to in their workplaces, and the exposures lead to high doses of specific agents that can affect them. We need to be aware of the potential hazards and we must know the risks such hazards can cause. For example, an x-ray technician is exposed to harmful ionizing radiation and must be protected from the source. A university lecturer is exposed to hazards of sitting all day in a desk \cite{owen2008too}. Also, working at a computer keyboard using repetitive tasks would predispose such a person to carpal tunnel syndrome. Therefore, we can use the principles of environmental epidemiological studies to explore what hazards exist that lead to health risks. We can also conduct health risk assessment of the workers or people employed in different occupations to further characterise the risk.
Having said that, we must be aware that there are some differences between people who are exposed to agents in the general environment (indoor or outdoors) and workers who are exposed to specific environments in their workplaces.
First, workers tend to be exposed for longer duration and higher intensity of exposure to whatever they are exposed to in their workplaces. Consider yourself as a "worker" and say you are exposed to dusts in a classroom to which you are allergic. Your average level of exposure will be higher during the term time than during other times of the year. The same logic applies to any worker who is working in a workplace where he is exposed to a hazardous substance or risky behaviour that he or she is about to undertake. A bus driver for example is exposed to longer hours of diesel exhausts when he is on duty than when he is not.  Second, we can argue that Workers tend to be healthier and younger than the members of the general public. The average work age starts in young adulthood and most people retire from work around the age of 60+, whereas the general age range of the members of the public are wider. More people in the younger age band would be working than older age bands and there may be difference in the nature and style of work and therefore exposure. We need to keep this in mind.  Third, for environmental health, there are different regulatory agencies for environmental and occupational health that look for exposure in the overall environment. For occupational health issues, there are rights and claims processes, hence there are specialised government agencies are different for Environmental health and occupational health issues. This distinction is not trivial as this tells us that the priorities are different to the governments for people in the general environmnental settings and people who are in work settings. Fourth,  insights gained in occupational health settings helped to solve problems in environmental health settings and each other. For example, lead exposure leading to health effects and neurological effects led to control of environmental lead.  Similarly, knowledge from environmental exposure can lead to investigation of occupational exposure; for example agricultural work, work in forestry and logging, etc are practices where work conducted in an outdoor environmental setting is also associated with occupational issues. 

Concept 3: occupational hazards are unique to occupations

Every occupation and workplace is associated with hazards. As we discussed earlier, hazards are those that can cause potential harm and when that harm is "realised", the health effects are risks. As in all occupations and workplaces, we work with some exposures that have their own physical and chemical properties, we exposed ourselves in many ways to physical forces that impact on us. For example, imagine a waiter carrying a tray of food to patrons in a restaurant and walks on a floor that has slippery surface. He slips on the floor, falls, and injures himself, and also in the process some food items and spices in the food hit his eyes. Now, several things occurred: (1) he slipped but because the surface was slippery and the friction forces were so low that he could not hold himself; (2) the impact of his fall was enough to injure his hip, but that was a health hazard; (3) the spices in the food that were uncovered led to splashing of hot liquid and spices to his eyes. So here is a case of mix between what is referred to as safety hazards and health hazards in this particular case. In safety hazard, we talk of energy transfer (friction, hitting a surface, hot surface, etc); in health hazard we talk about risk of health effects due to exposure to some other exposure. 
Safety hazards. -- In an unsafe workplace, energy gets transferred from the source to the recipient, the worker. For example, the waiter who slipped on the slippery floor had loss of friction force and his movement disrupted his balance and he crashed on the floor. In other situations, other forms of energy such as chemical energy get transferred from a chemical body to the humans. For example, firefighters who fight chemical fires such as that resulting from acid fires should not aim to douse the fire with water, if they do, they may get burnt from the acid fumes; an electrician wears insulated materials to work and turns off electric current sources to avoid electrical shock. From the source, uncontrolled transfer of energy (physical, chemical, electrical, thermal, kinetic, radiation) happens to vulnerable recipients. Example: electric shock, touching a hot plate, lead from battery units, radiation from Xray units, hit by a moving part in a car manufacturing unit, etc. A safety hazard and the resulting health effect (fractured bones, or electrocution, or burn) are unique to occupational health.
Health hazards. -- In contrast to safety hazard that is largely preventable, a health hazard is one that occurs after a latent period. A worker is exposed to a health hazard for some time and the hazard accrues for some time before manifesting in health effects. We discuss five types of hazards: chemical hazards, where specific chemical compounds will lead to either acute or chronic health effects. For example sudden exposure to high concentrations of carbon monoxide can lead to death due to increased accumulation of carboxyhaemoglobin that prevents transfer of oxygen to cells; continued exposure to asbestos can lead to mesothelioma. Physical hazards are the ones where persistent exposure to physical agents (such as noise or vibration) can lead to illnesses. For example, if gardeners do not take precaution while operating chainsaws or hedge trimmers using gloves, or ear muffs, then they risk damages to the median nerve in hand, or hearing loss (noise induced hearing loss). Clerks and writers, if they sit for long time at a place risk developing a number of health effects (sitting is the new smoking).
1. Chemical hazards. -- exposed to chemical agents at work and develops disease
2. Physical hazards. -- Noise, pressure, vibration, non-ionizing radiation
3. Biomechanical. -- Heavy weight lifting, repeated movements (carpal tunnel syndrome)
4. Biological. -- infection from infected syringes, HIV, Hepatitis B, etc
5. Psychosocial. -- Job loss, stress

Concept 4: Statistics of NZ occupational health

Driscoll, Pearce and colleagues (2004) have tabulated and discussed several disease conditions that occur due to workers are exposed to specific objects and agents in the workplace \cite{driscoll2004burden}. Other sources of data to understand the burden of occupational diseases in New Zealand are from Worksafe website data section and from the ACC (see https://www.acc.co.nz/about-us/statistics/); Here, see a part of the picture of the occupational health issues in New Zealand. What do you think?
Prevalence of Hepatitis B among New Zealand workforce: