Early in the history of hay asthma heat was considered its chief cause, Bostock, its first describer, having held that view, as have also many of his successors. It is now generally conceded that heat of itself is not a cause, although by promoting vegetable growth and causing dust it may still be regarded as an indirect factor in its etiology. That heat of itself is not a cause is proved by the occurrence of the disease not during the intensely hot weather of midsummer, but in the late spring and early fall. It, however, undoubtedly produces a temporary aggravation of many of the symptoms. This appears to be especially the case in the autumnal variety, as those who have the disease in the spring seldom complain of any ill effects from heat.
"Strong light, sunshine, especially when it falls upon the face, will produce a violent paroxysm of sneezing, and the other symptoms then follow in quick succession; and moving from shade to sunshine, even when not otherwise annoying, will do the same." This is the opinion of Wyman, and coincides with that of Phoebus, Abbott Smith, and others, and is amply confirmed by the experience of the writer. This applies also, though in a less degree, to artificial light, especially gas-light.
Dryness of the atmosphere, by promoting dust, may be regarded as an indirect cause. Hay-fever patients agree almost unanimously that their symptoms are aggravated on clear, bright, dry days, and that they feel most comfortable in damp and cloudy weather.
There is no evidence to show that electricity is in any way connected with the etiology of hay fever.
Ozone is certainly not a cause, as hay-fever patients feel best on the sea-coast and ocean, where ozone is most abundant.
Long before hay fever was recognized by the medical profession hay was supposed by the general public to be the cause of the disease. In England especially, but also in the north of France and in Switzerland, this opinion prevailed very generally. Some suppose that the dust which it contains is the real cause, while others attribute it to its peculiar odor. In those susceptible to its influence it appears to make but little difference how they come in contact with it, whether in an open field where it is mowed, by driving behind a wagon loaded with it, or by entering a stable or loft where it is stowed away. It is not, however, the cause of the autumnal variety, as it is harvested in the temperate regions of North America, where this form of disease is most common, in June or early in July, which is six or eight weeks earlier than the period at which the attacks commence. That hay is a cause of the earlier variety of the disease is evident from the experience of numerous intelligent invalids, who trace it to that agent from the fact that the outbreak coincides with the blooming or harvesting of hay, and that removal from the locality in which they are exposed to its emanations is followed by relief. It must be remembered, however, that hay does not consist of dried grass alone, but that it contains other plants and flowers, as well as a large amount of dust.
The flowers of grass, especially those of the Anthroxanthum odoratum, may be regarded, like hay, as one of the causes of hay fever—a fact that was early recognized by Gordon and others. Blackley12 cites the case of an Indian medical officer of high rank, whose statement is as follows: "I have suffered from hay fever for about thirty-five years; I have had it both in India and in England. The period at which the attacks come on is not fixed, the date of the attacks depending more on the grass ripening late or early than on any other circumstance. They always begin toward the end of the hay season, when the grass is fully in flower, and cease slowly and gradually—not directly—on gathering in the grass."
12 Hay Fever, its Causes, Treatment, etc., p. 47, London, 1880.
Rye, oats, and wheat in bloom may also be ranked among the exciting causes of hay fever.
Indian corn in bloom often causes symptoms of hay fever, but that it does so only in certain cases is evident from the fact that the disease does not exist in some places where large quantities of corn are raised (Wyman).