Since man (and every other animal) has had dioxin in their system at very low levels during their evolution, these low levels are natural to the species. However, the ambient levels of dioxin have risen in recent time due to the activities of man, in most part due to combustion processes. No studies that I know of have addressed whether this has had a deleterious effect and if so, to what extent. Common sense, however, tells me that it probably is not a good thing.
To reduce ambient levels would require either curtailing man-made combustion, or treating all man-made combustion products before they are emitted. I don't think this is a possibility in the near future. However, the efforts to curtail global warming are a step in the right direction, IMHO.
Fortunately, plants to not take up dioxins. I was involved in an early study to determine this. We synthesized radiolabeled tetrachlorodioxin for the US Department of Agriculture for use in a study they did on uptake of dioxins in several plants. Since the analytical methods did not exist at that time to chemically determine dioxins at low levels, they could track the radioactivity to find the answer. They found that corn kernels, for example, contained no detectable dioxin. Root plants, however, had dioxin on the surface of the roots which could be washed off. My advice is to thoroughly wash root vegetables before eating.
Since animals bio-concentrate dioxins they are exposed to, limiting certain of these in the diet would be beneficial. Fish live in water which contains dioxins at very low levels. Their bodies trap a portion of them in their fatty tissues raising the level in their bodies about a factor of 3000. This takes time. Fish which are long-lived in general contain higher levels of dioxin. The US government has advisories on certain waterways against eating more than a specified number of fish per month. Also, large game fish likely have higher levels. Plant-eating animals are not exposed to equivalent levels of dioxin.
There have been several incidences of animals being contaminated with dioxins due to contaminated food. One incident mentioned in the comments above was chicken found to be contaminated with dioxin during a routine food analysis supermarket survey by the US government. The source of the contamination was due to the presence of contaminated clay which is put in the food to facilitate its ability to flow easily in the machinery which transfers it (usually via augers) from place to place during the feed's manufacture.
I was involved as a consultant to a Houston law firm in the litigation surrounding this incident. The clay contamination was of natural origin. Clay is made during the deposition of silt in lakes. It is thus stratified and can act as a 'time machine' for determining substances in the water at the time of the deposit by analyzing core samples. The deeper the sample, the older it is, and the time period can be back to prehistoric ages. The pattern of dioxins in the clay, mined from a depth of about 30 feet, was unusual. It did not match any known source.
It was found that a species of algae could manufacture dioxins. The algae were deposited, along with the silt, in a layer 30 feet below the top of the clay deposit. Since the contamination was of natural origin, and the possibility that clay could be contaminated in this way was was unknown, there was no culpable party, and the law suits evaporated. We are fortunate that our government had the foresight to monitor the food we eat for dioxin contamination.
Personally, I do not worry about dioxins in my diet. Man has evolved in the presence of dioxins and can handle the 'normal' exposure encountered in his daily life. What the government study is trying to do is to determine the level of concern for unusual exposure to dioxins. This, in turn, allows them to control the populace's exposure to dangerous levels of dioxins.
I hope this answers your questions.
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