What is the cause and solution for the “environmental illness” my client described?

As you saw intuitively, this is a kind of parallel of the immune system in how it operates. That is a system of chemical sensing, detection, and provision of a contravening chemical response in the generation of antibodies, the stimulation of immune cells to release things noxious to microbial invaders, and so on, the detailed workings of immunity that protect a person against infectious organisms. There is a parallel response by the emotions when subjected to perceived threats of any kind. Memories of that trauma will be stored and can be revivified with a full re-expression of the emotional consequences. Trauma will register on the body, being stored as a memory containing the emotional consequences that can haunt a person, and be re-experienced again and again and again leading to a state of chronic anxiety. This is an automatic process that will not be understood on a conscious level but will simply be experienced as angst, a kind of chronic state of uncertainty, a seeming worry without a cause, and many times the mind will latch onto things as a way of wanting to understand and cope with being out of balance and feeling unhappy. There can be a further physical corollary that is generated by the body, in re-experiencing the stored trauma memory and its consequences, because the body responds to emotion and there can be all kinds of links created to cause things to happen within the physical function that get noticed consciously. For example, there could be tightness in the chest, even difficulty breathing, cardiovascular changes to ramp up the heartbeat, increase blood pressure, and even send someone into a panic attack with a fainting spell. There can also be a carryover in having the threat of something perceived as a menace that is noxious, learned by the body to rebel and reject it, and that might well recruit the immune system to mount an attack. This is the basis of asthma, producing symptoms in the lungs through an attack on things breathed in with the air that are foreign, and not necessarily of any danger but misinterpreted as dangerous. The same is true of the so-called "environmental illness" phenomenon where a chemical odor or even contact with chemicals on surfaces might trigger a very strong physical response with symptoms of varied kinds. This is attributed merely to ultrasensitivity to chemical substances, and that is a correct definition and understanding as far as it goes. But it is linked with memory, and oftentimes the memories are not even from the current life but from other lifetimes lived, and the body is being sent a warning by the deep subconscious reviewing the akashic records of prior trauma. And the body will dutifully reenter the fray and try to battle an old adversary that is not truly present but it interprets the cry of battle to be a kind of marching order to stir up a defense, and that can cause harmful changes in the body, just as with an overactive immune system producing collateral damage on normal cells and tissues and not just pathogenic organisms it is trying to kill. So the answer for environmental illness is a kind of detoxing of the memory bank to decouple exposure to various stimuli from this amplification mechanism to turn it into a five-alarm signal resulting in an overkill. That is best done through the process you are using for this client, of Deep Subconscious Memory Reset, because the experiences leading to the sensitivity in question can be worked on directly to remove the associations that are on file and make the prior exposures neutral in their impact rather than emotionally charged. So it does not change the record of what happened, but it does change the consequences, to make them be seen as neutral and not deserving of a reaction any further, and that will end the problem with enough session work to pare back the storehouse of associations underlying the condition.