Post Traumatic Stress Disorder (PTSD) impacts up to 6% of the population . It is a severe anxiety disorder that can occur in some people after they experience a severe psychological trauma or ongoing psychological "toxic" stress. It results in flashbacks, unpleasant intrusive memories, mood and behavioural changes, and can also result in many unpleasant physical symptoms, as well as depression.
Post Traumatic Stress Disorder (PTSD) is now being linked to central sensitivity syndromes like fibromyalgia, chronic fatigue syndrome and multiple chemical sensitivity,  Although additional research regarding the link between PTSD and central sensitivity is necessary says Dr Yunus.
Like people with central sensitivity syndromes, people with PTSD experience HPA axis dysregulation. This causes ovreactivation of the fight/flight fear response and makes it diffficult for the nervous system, endocrine (hormone) system and immune system to work efficently together. This slows or impedes the restoration of balance (homeostasis) after exposures to a real or perceived stressor e.g. physical injury, psychological upset, toxic injury from overexposure to mould or chemicals, infection, food intolerance, poor indoor air quality etc. When people are not in balance they can experience a wide range of physical, psychological, environmental and stress related symptoms.
People with PTSD also experience changes to the brain , nervous system, that amplify pain, stimuli and stress sensitivity. This is particularly severe if the person is also experiencing hypocortisolism says Fries 2005. Which apparently impacts around 1 in 4 people with PTSD, who may also have co-morbid fibromyalgia or chronic fatigue syndrome. 
People with PTSD can benefit from addressing the common causes of HPA axis dysregulation,. This is done by following a basic Self Care - Self Management Plan like the one recommended by 50 symptoms gone here, But it is best if the plan also includes ongoing psychological support, desensitisation therapies, and self help strategies specifically aimed at helping people with PTSD, like the Self Help Strategies for PSTD recommended by Anxiety BC. (download - selfhelpptsd)
Like Multiple Chemical Sensitivity (MCS), PTSD is a time dependant sensitisation disorder except that it is the emotional stress (instead of chemical) that causes sensitization by a single severe exposure or repeated ones to various stresses, eg, war, torture, childhood abuse, rape, natural disasters, and terrorist attacks . Startle reflex (by a loud auditory tone or noise and manifested by eye blinking and increased heart rate) is exaggerated in PTSD 
A small percentage of people with PTSD also have Multiple Chemical Sensitivity. This appears to be the case in some Gulf War Veterans who were exposed to high levels of pesticides while serving in the Gulf. 
If the person with PTSD also has Multiple Chemical Sensitivity (MCS) both conditions need to be managed simultaneously, which can be a bit tricky. Also reducing exposure to stimuli like noise may provide some symptom relief.
Sometimes veterans who have experienced a traumatic brain injury caused by a concussion or exposure to very loud noises like explosions are misdiagnosed with PTSD. TBI and PTSD can coexist. 
As you can see PTSD is a very complex disorder. Which is why it is advisable to work with a mental health professional who is specifically trained in the management of PTSD.
To find out more about PTSD I recommend you read the article: Post Traumatic Stress Disorder (PTSD) overview by Healthy Place - America's Mental Health Channel or the PSTD fact from Beyond Blue.
4. Fries 2005 A New View On Hypocortisolism.
6. Shalev AY, Peri T, Brandes D, Freedman S, Orr SP, Pitman RK. Auditory startle response in trauma survivors with posttraumatic stress disorder: a prospective study. Am J Psychiatry 2000;157: 255-61.
7. Gulf War Illness and the Health of Gulf War Veterans: Research Update and Recommendations, 2009-2013 http://www.bu.edu/sph/files/2014/04/RAC2014.pdf
8. Bryant 2011 Post-traumatic stress disorder vs traumatic brain injury http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3182010/