House dust mites -
The British Medical Journal of 24 October 1998 published a statistical analysis of objective trials of house dust mite control and concluded that there was no evidence of benefit to patients whose asthma was attributed to house dust mites. Before embarking on a lot of expense and trouble you may want to know what this report has to say and whether it should be regarded as reliable. A new web page discusses this and contains links to the original article and accompanying comment in the Journal.
How can we get rid of mites? General points.
What should we do to get rid of mites and their dust? The practicalities.
DISADVANTAGES OF MITE CONTROL. For many people the cost may be prohibitive. Becoming obsessed with mites is also bad.
Other sources of information about mites including a video and a book.
The answer is not an easy one, but we think it is important for you if your asthma or nose allergy is caused by allergy.
Infantile eczema or its equivalent in older children and adults may get better in some cases through avoiding mites and their dust. There is other evidence that mites may be involved in causing it. In some people, especially younger children, food allergy may wholly or partly cause eczema. But we don't know as much about this area as we would like.
What are house dust mites?
Few people nowadays have not seen a television programme about house dust
mites. Newspapers are full of advertisements for things which are supposed to
help you get rid of them and the nasty dust they leave all over your house.
House dust mites are tiny (up to 0.3 mm) animals related to ticks and spiders and live in housedust. There is not a house without them, but some houses contain huge numbers and other houses contain almost none. This does not only depend on cleanliness, but depends very much on the amount of moisture in the house; dry houses in very cold climates or on high mountains have few mites, but houses in temperate climates and normal altitudes have more.
House dust mites eat the dust which comes from our skin all the time.
They leave droppings everywhere they go. Their droppings contain left-over enzymes which the mites use to digest the skin dust. It is these enzymes which are the most important part of mite dust in causing asthma and other allergic diseases.
In fact, house dust mites and their droppings are the most important cause of asthma worldwide.
There is ample proof that living in surroundings with little or no mite dust improves or cures asthma in those people whose asthma is caused by it.
In practice it is proving to be almost prohibitively difficult for most people to achieve this.
There are useful things you can do. It is also worth being aware of things people will try to sell you which may be a waste of money.
Doctors who do not have special training in allergy are likely to have more difficulty. Their knowledge will not be so up to date or complete, so the chance of getting wrong advice is higher. But no doctor, however clever or specialised, is always right.
You can also tell to a fair extent for yourself, if there are enough clues in your case. Here are some things to look out for:
One reason for this is that mostly it is not enough to halve the amount of mite material in your house. Worthwhile results mean cutting the house dust mite pollution to BETWEEN ONE TENTH AND ONE HUNDREDTH of what it was before.
But it can be tremendously worthwhile.
A word of caution. We breathe mite dust in other places than at home. No matter what you do to your house, dust elsewhere may still cause trouble. So how successful you are may depend on how important mite dust from sources outside your home is in your case. This will vary a lot from person to person. This aspect is a focus for current research, and we may know more about it in a few years.
Another word of caution: do not make the treatment worse than the disease. A few people get an obsession about cleaning the house and cripple themselves socially in an attempt to avoid mites. This is a terrible result of well-meant advice. Do please rely on sound advice from a recognised expert. The object of mite control should be a more normal life for the member of your family with asthma or allergy and for the family. Click here to read more about this.
It remains contentious whether the good results of house dust mite avoidance achieved in residential clinics can also be achieved in one's own home with methods now available. In the summer of 2003 the New England Journal of Medicine published two important sets of results which show that merely issuing mite-proof bedding covers could not be shown to be helpful. It has to be said however that the main advocates of mite avoidance had not advocated bed covers alone. It remains possible that more extensive control measures may help, particularly over a much longer period than that of the trials. An accompanying article makes this clear.
Some housedust mite sensitive people are in a desperate situation and want to leave no stone unturned. Others want to do whatever they can to reduce reliance on drugs. These are legitimate views. There are people who wish to try mite control even in our unsatisfactory state of knowledge.
However, no-one thinks mite control will help if your child or you are not in fact made ill by house dust mites. This means firstly being allergic to them (usually checked by skin or blood tests) and secondly having symptoms caused by this as a result. Not everything which causes a positive test result also causes disease.
If you want to try mite control, you are much more likely to avoid mistakes if you consult a recognised allergy specialist.
It is common for people with house dust mite allergy to notice no benefit, despite their efforts. This may well be because they cannot manage to do enough. Once again, discussing it with an expert may help you to judge your chances of success and avoid an expensive approach which has little chance of helping you because of your circumstances.
But there is proof that mite avoidance does work in some situations.
You will get surprisingly different advice from different sources. Some of the differences are due to our uncertainties and a genuine difference of opinion between experts because we don't always have real proof for everything. But other differences may be due to inexpert advice and the greed of people who want to sell you expensive things whether those things help or not.
Here is my current list of recommendations:Top of page (Table of Contents)
If washing at 60 degrees centigrade would damage a washable item, another suggestion is to freeze the item for 24 hours to kill the mites and then to wash at a lower temperature. This sounds as if it should work. If you have the space and money you could get a small freezer for the purpose. This idea comes from staff of the National Asthma Campaign in the UK.
If you do not remove the carpet from the bedroom, take care to avoid contact between bedding and carpet.
Many people recommend putting furry toys into the freezer for 24 hours to kill the mites. There is no evidence that this works, and I would not expect it to. Freezing leaves the harmful dust unchanged, and new mites will walk into the furry toy as soon as it touches the bedclothes again. It might perhaps help if you also wash the furry toy in the washing machine to remove the dust which the mites have made inside it. But we have no idea how effective such washing would be and how often you would have to do it. At the very best all this will be less effective than not having the furry toy in the bed.
Every time someone walks over a carpet, mite dust is spread into the air, and will take 2 hours to settle. Babies and young children crawling on the carpet are surely especially at risk.
For practical purposes wall-to-wall carpets cannot be cleaned adequately, though steaming carpets thoroughly may help.
Replace carpets with sanded and varnished floorboards, or a vinyl, linoleum, tiled or purpose-made wooden floor. Use a minimum of scatter rugs and wash these several times a year. In cold climates, hanging the rugs outdoors in freezing weather helps.
If you live in a flat, you may be forced to have carpets to reduce noise affecting your neighbours. Your lease or purchase contract may even specify the thickness. The best you can then do is to have an excellent vacuum cleaner, and you could apply mite-killing chemicals from time to time (but tests don't show they work in practice) or use other special mite-killing processes, do everything you can to reduce humidity in the flat, and cover the most walked-on parts of the carpeting with rugs which can be washed or cleaned outside the flat. Give a higher financial priority to replacing the carpet more frequently, and indeed consider getting a lower quality of carpet and replacing more frequently rather than going for high quality carpet at great expense. See if you can get away with or negotiate a deal to remove carpets from bedrooms in the flat, using washable rugs on the main areas you walk on and always walking in socks, or really soft slippers such as mocassins, or even on bare feet when walking on the smooth floor rather than wearing any kind of shoes, however soft. Your good neighbourliness may persuade your neighbours to be merciful to your lungs. Despite our misgivings about air filters, they may be worth a try in this difficult situation.
(The above is the majority view among experts, but there is another view. Dust on smooth floors blows up into the air more easily than dust from carpets. It is dust in the air which does the harm; dust in the carpet is harmless as long as it stays there. A minority of specialists argues that carpets are therefore better than smooth floors. We disagree, but there is no published research to prove which approach works best for patients.)
But there's a catch. You have central heating, and you probably don't want to have a cold house. So you are not going to want to open windows more than you have to. The obvious answer is to dress more warmly and to allow your house to be colder, but you and your family may not want to do that. There is however an advantage to this; at lower temperatures the mites are less active and breed less fast. Keeping your house cooler will reduce mites, especially if ventilation keeps the humidity down.
But what if you want to be warm and to ventilate your house at the same time without paying the earth in heating bills?
Heat exchangers are claimed to be a way of doing this.
For dehumidifiers to work well for house dust mite control in the UK climate, more powerful models than those commonly on sale for domestic use will be necessary. In the USA, Therma-Stor Products (in the USA, Tel. 1-800-533-7533) promotes this idea and has the equipment to match. They would probably supply in the UK as well, as they do have a related company here.
Rarely used sleeping bags can be terrible too. Dry clean immediately on return from each journey, then store in plastic bag without holes. For the cost of dry cleaning you could almost get a new cheap sleeping bag. But this does work.
The evidence:Covering bedding with special-purpose dustproof covers is necessary whatever your pillows are made of.
Kemp, TJ & others, "House dust mite allergen in pillows". BMJ 1966;313:916.
Crane J & others, "Increased house dust mite allergen in synthetic pillows may explain increased wheezing". BMJ 1967;314:1763-4
However, vacuuming will remove some of the skin dust on which the mites feed, and a little of their droppings, and may still be worthwhile as part of a proper plan.
The allergic person should not do the vacuuming, and should not be around whilst the vacuuming is being done. Nearly all vacuum cleaners, even those with good filters, take dust from surfaces and disperse a surprising amount of mite dust into the air.
Machines do differ a lot in how much dust they allow to get into the air.
If your vacuum cleaner is old and leaks and is not likely to be cured thoroughly by a good repair, replace it; see below.
If you have a good vacuum cleaner and the asthma sufferer is out of the house when it is used, it may well not be worth prioritising the purchase of a special machine. If the housewife is the asthma sufferer, the first priority is to shift the task of vacuuming to someone without this problem, but if that is not possible, a special vacuum cleaner should be a high priority.
Brands are constantly changing. General advice in the UK: avoid water-filled cleaners, which spread aerosols of dust. Do not spend more than 500 pounds sterling on any machine, however convincingly sold. There is a good range of reasonable machines at around 200 pounds sterling, though this is much more than many people spend on a normal cleaner. Some machines in the 200 to 500 pounds sterling range may have some advantages. Seek expert advice if you can get it (difficult, but try the British Allergy Foundation or the Consumers Association in the UK). Buy any reputable machine with an excellent filter.
The main cyclone-based bagless vacuum cleaner currently on the UK market (Dyson) is good in some ways, but the bagless design means there is a problem about emptying it without spreading a lot of dust. Even if you do this outdoors, a cloud of dust is liable to cover your clothes. Any mites in this dust will be spread to anything which comes into contact with the clothes you were wearing. I have purchased the latest model, with added high-grade filters, and am disappointed that the emptying problem has not been addressed. Wearing overalls to do the emptying might be better but from other evidence I would not assume this will be a complete answer.
Central vacuum cleaners are an excellent idea. The dust is piped to a container in your garage or a similar space, outside your living space. With a good system emptying will be infrequent, reducing an important form of dust exposure and surely eliminating the need for the allergic person ever to do the emptying. I do not know how well these systems replace the sweeping and sucking action of upright vacuum cleaners. Installation into a house whilst it is being built is easier than in an existing home, but both are possible. I have no direct knowledge of any of these systems. Click here for a fuller explanation from a supplier in California, and an idea of prices.
Do not imagine that any vacuum cleaner will be perfect. There is a limit to what any vacuum cleaner, however good, can achieve. Getting rid of carpets is better.
The suppliers claim that the saving in heating bills will pay for the running costs. Of course that must be on the basis that you would ventilate your house if you didn't have a heat exchanger. But it is healthy to ventilate, and you should be doing so.
You can get heat exchangers for a single room, which look a bit like a large air-brick from the inside. Or you can get versions for just the upper floor of a house, or for the whole house, which have the pump and heat exchange box in the attic, with ducts going to the rooms below.
Heat exchangers lower the humidity in your house, which is one way of reducing mites. They do not lower the temperature, which is another way. But you may be a lot happier with a warm house.
Although house dust mites feed on skin dust, an expert quoted by the manufacturer claims that they also need a microscopic fungus which grows in house dust. The fungus is called by the latin name Aspergillus repens and can also not grow without enough moisture in the air. The arguments for a house which is dry to a healthy level seem to be adding up.
Another advantage of heat exchangers is that by fighting condensation they should reduce any chance of other moulds (fungi) growing in your house, not just the fungus which the mite needs. Some people with asthma can be very sensitive to spores from moulds reaching their lungs. You can kill mould quite easily with disinfectants, but if a wall or window frame regularly gets damp from condensation, the mould will keep coming back. Drying the house to healthy levels by eliminating condensation will work much better. Getting rid of moulds by keeping the house dry is definitely good if you have asthma.
For low humidity to have a serious impact on mite numbers, experience from other countries suggests that the relative humidity has to drop to 40% or below for weeks or months. It seems to us that you are unlikely to achieve this in Britain by using a heat exchanger. In summer outdoor and indoor temperatures are similar in a climate like that of the UK, and outdoor humidities are almost always too high to cause mites any harm. They may still be lower than indoor humidities much of the time because of indoor water vapour production. In our opinion there is not enough information to comment accurately on the effect of heat exchangers on health in these conditions, though on the basis of expert opinion that more ventilation is a good idea, they ought still to deserve recommendation.
Our current view is that drier is better within the humidity range you are likely to achieve, but that it is unclear whether the heat exchangers will achieve enough in a temperate climate to make a noticeable difference to health on their own.
Heat exchangers sound like a good idea, but more research is necessary.
A British manufacturer of heat exchanger ventilation installations for homes is Baxi Clean Air. Click here to find out more.
To understand why, you have to understand something about the things which add dust into the air and things which remove dust from the air. The concentration of the dust you breathe will depend on the balance between the two. The effect of filters depends on their overall contribution amongst the other things which happen.
Mite dust particles are larger than cat dust particles, and fall out of the air more quickly. So mite dust concentrations in a room are much more dependent on human activity in the room, stirring up new dust into the air. An air filter appliance will itself stir up the air in a room and thus delay the settling of mite dust. Free-standing filter appliances are by their nature somewhat inefficient because progressively they start to filter air which they have already filtered before. These facts mean that such a filter often makes little difference to the disappearance of mite dust from the air in an undisturbed and unventilated room and cannot keep up with the raising of mite dust in a disturbed room.
In contrast, cat dust is much finer, and stays in the air much longer. This means that an air filtering appliance can make a difference to airborne cat dust in an undisturbed room, though probably not much difference in a room where new cat dust is being raised by humans or cats. However, installing an air filter is not a reasonable substitute for getting rid of the cats.
So far, this ignores the effect of ventilation of the room. Exchanging the room air with outdoor air (e.g. by opening windows) could easily outstrip the effect of a room air filter, at the price of making the indoor temperature more like the outdoor temperature. If temperature or noise are not a problem, ventilation is likely to be a much more effective answer than filtration. If temperature is an issue, then a heat exchanging ventilator may still be a much better answer than a free-standing air filter, with the potential for a bigger effect for smaller running costs and noise levels. All the experts on mite dust control recommend ventilation of the house. In the presence of reasonable ventilation, a free-standing air filter will not make a worthwhile difference and will surely not justify its cost.
They are essential for mite control. There are quite a number of kinds. Good ones are expensive. To be effective and good value they must be:
The types available are:
How to recognise this type: you can see woven or knitted cloth on one side, and a shiny or dull plastic surface on the other side.
How to recognise this type: you can see an exceptionally fine woven pattern on both sides. The pattern is extremely fine even under magnification.
Recent research (Sporik and others, Journal of Allergy and Clinical Immunology, April 1998, vol. 101, pp451 -456) results show that these covers are effective and remain so for at least a year. The effectiveness of the non-woven fabric was not compared with that of other materials. We still do not know how well they will function after years of use or after repeated washing.
There is no national or international standard by which consumers can judge the quality of these covers, and some seem to have been of inadequate construction or quality. How to recognise this type: the paper-like material consists of fibres bonded together to make a sheet. The surface may be patterned in some definite way, but looks different from any woven or knitted cloth.
How to recognise this type: much thicker than the other kinds, with a thick synthetic felt on the underside and a woven cloth on the side nearest the sleeper. Soft to the touch and not waterproof or air-proof, since it does not need to be.
Taking mite control seriously is a major step in your life. It involves a major re-think of your lifestyle and a financial commitment perhaps comparable to building a new extension to a house. Planning ahead makes it less wasteful financially; you may not be able to do much now, but planning changes when you move house can save you from expensive changes later. Many experts are passionately convinced about the value of mite control, but real proof that it works for most people in practice is scantier than it should be.
Our verdict? If you have bad asthma, nose trouble or eczema, AND IF YOU HAVE BEEN PROVED TO BE ALLERGIC TO HOUSE DUST MITES, go for it.
If you have bad asthma, nose trouble or eczema and have not had positive test results against house dust mites, consider it if there is a balance of probability that the mites are responsible, but be aware that you may be wasting your money and effort. For some people this can be a good bet, but you have to be prepared to find you have wasted your time.
This page is maintained by Martin Stern (firstname.lastname@example.org)
'non-allergenic pillows' may be worse, and other changes; 1 Feb 1998,
central vacuum cleaners, added comment on Dyson, Therma-Stor dehumidifiers, tight weave bedcovers 6 April 1998,
changed info. on heat exchangers 9 April 1998, ADM changed to Baxi 22 April 1998
Tyvek (plastic nonwoven fabric) bedding covers now recommended, washable cotton duvet, 25 May 1998.
Non-woven bedding covers; updated, 27 Jun 1998.
Much expanded explanation of why freestanding room air filters probably don't work well, 21 August 1998.
BMJ review claiming mite control is useless for asthma, 24 & 26 Oct 1998.
Minor changes 6 Feb 1999, 22 feb 2000, 9 Mar 2000, 7 Jan 2001, 5 Apr 2003. Bed covering alone is unhelpful; 31 July 2003.