In this context is the suggestion that ME is caused by environmental factors, triggered by environmental factors or exacerbated by them?
Presumably until we understand the causal mechanisms underlying ME we can not begin to answer this question.
Personally I don't believe that my ME is caused by external environmental factors as its onset was contiguous with an acute episode of glandular fever (Epstein Barr), though I understand there may be multiple causes and recognise that some people believe the main cause of their ME is environmental, eg mould.
However, my symptoms can be triggered by environmental factors, such as getting too cold, eating gluten or any form of stress physical, pechological or emotional. So I can be in relative remission but environmental factors can trigger a relapse.
This also raises the question as to how you could distinguish an initial cause and a subsequent trigger. For example the theory that an acute infection triggers a particular gene expression that then could result in symptoms being triggered by other factors.
Perhaps the most confusing aspect in this context is PEM. 'Doing too much' triggers or exacerbates symptoms, so for some people if they are able to manage their activity levels they are able to manage their symptoms too. For me it is external factors that most commonly force me to 'do too much', financial, organisational or family events. Do these count as environmental, things in my environment causing my PEM? However, more recently as my condition has worsened the level of activity triggering PEM has dramatically reduced and basic activities of daily living can trigger it.
Also for me ongoing symptoms can be exacerbated by environmental factors be it noise, chemicals or temperature. It is not always clear if this is a general hypersensitivity (eg light and noise) or a specific response to specific stimuli (eg gluten intolerance).