There are many things around the house that a person can do to lower the risk of asthma. Get rid of old carpets, foam mattresses, and gas stoves.  Use only feather pillows.  Disinfect pillows if possible.  Don't use a humidifier.  Use a dehumidifier if you house is damp. 

Carpets: Old Carpets, Moisture, Mold Good for Dust Mites; Bad for Humans: Dust from bedrooms in 485 houses over one year found high dust mite allergen Der p 1 in the floors (17.2 microg/g fine dust) as well as in the beds (20.3 microg/g fine dust) with lower levels in winter and spring. They were higher in houses built before 1980 and those with central heating, weather board walls, damp bedrooms or fitted old wool carpets. Der p 1 levels in bed dust were significantly higher in houses built before 1980, with wooden floors built on stumps, with high relative humidity, with visible mold in the room, in beds with an old mattress or in beds without a quilt. Residential characteristics influence Der p 1 levels in homes in Melbourne, Australia. Dharmage S, Bailey M, et al. Monash Medical School, Victoria, Australia. Clin Exp Allergy. 1999 Apr;29(4):461-9

Carpets: Old Carpets Bad; No Carpet or New Carpet OK: In the homes of 474 8-10-year-old asthmatic children, dust mite allergen Der p 1 levels were significantly higher in households with older carpet than households with no carpets or newer carpets, and higher in the autumn. Households with three or more children had higher levels of Der p 1 than households with fewer children. Bedding levels were significantly higher in beds with kapok or inner sprung mattresses, or wool underlays and at relative humidities above the mean (51%). However, the type and age of floor covering appears to be the single most important factor. Determinants of house dust mite allergen in homes in Wellington, New Zealand. Wickens K, Siebers R, et al. Wellington School of Medicine, New Zealand. Clin Exp Allergy. 1997 Sep;27(9):1077-85

Carpets: Fitted Carpets Good; Mold Bad for Asthmatics: Measurements of dampness, mold exposure, and house dust mite levels and adult asthma in 38 study centers from the European Community Respiratory Health Survey found that fitted carpets and rugs in the bedroom were related to fewer asthma symptoms and bronchial responsiveness (OR range, 0.69-0.91). Reported mold exposure in the last year was associated with asthma symptoms and bronchial responsiveness (OR range, 1.14-1.44). Reported mold exposure was highest in older houses with recent water damage. Housing characteristics, reported mold exposure, and asthma in the European Community Respiratory Health Survey. Zock JP, Jarvis D, Luczynska C, Sunyer J, Burney P; European Community Respiratory Health Survey. Barcelona, Spain. J Allergy Clin Immunol. 2002 Aug;110(2):285-92

Carpets: More People Living in House Decreases Asthma Risk; Wall to Wall Carpet OK: Parental questionnaires for 2,193 children (73.6%) in Perth found children born in Australia had a significantly increased risk of current asthma (odds ratio (OR) 2.37, p = 0.001). Having a mother born in Australia was the only factor independently associated with an increased risk of current hay fever (OR 1.56, p = 0.005). Increasing numbers of people living in the home were significantly associated with a multiplicative decrease in risk of current asthma (OR 0.88, p = 0.03) and eczema (OR 0.82, p = 0.01). Houses made of fibrocement (OR 2.40, p = 0.02) and the presence of mats on less than half of the floor area in the "main bedroom" (relative to wall-to-wall carpet) (OR 3.50, p = 0.003) increased risk of eczema. Environmental factors and asthma and allergy in schoolchildren from Western Australia. Palmer LJ, Valinsky IJ, Pikora T, Zubrick SR, Landau LI. University of Western Australia, Perth, Australia. Eur Respir J. 1999 Dec;14(6):1351-7 

Feather Pillows Best: In a very small study of just 10 pillows - five feather and five synthetic - which had been used for between 18 months and 20 years, researchers found up to 16 types of fungi. Feather pillows had fewer species than synthetic versions, particularly in the case of a fungus which exacerbates asthma. The fungal spores found in the pillows fed off human skins scales and dust mite faeces. Fungal contamination of bedding was first uncovered by studies carried out in the 1930s, but few studies have been done since then. The researchers advised to disinfect pillows, but I don't know how you would do that. Maybe by using steam heat?  Ashley Woodcock, et al. University of Manchester. Allergy 10/15/05.

Foam Pillows, Gas, Dampness Bad; Cotton Quilts, Fruits, Veggies Good: 10,902 schoolchildren, all age 10, were tested. Factors during the first year of life and currently that were significantly associated with wheeze were cooking with gas (odds ratio 2.04), foam pillows (2.58), and damp housing (1.89). Factors protecting against wheeze were cotton quilts and the consumption of fruit and raw vegetables. Factors associated with difference in prevalence of asthma in children from three cities in China: multicentre epidemiological survey. Wong GW, Ko FW, Hui DS, Fok TF, Carr D, von Mutius E, Zhong NS, Chen YZ, Lai CK. University of Hong Kong. BMJ. 2004 Aug 28;329(7464):486. Ed: I found that most pillows for sale in U.S. retail outlets are polyester, not foam.  That's good.  However, I just discovered that a mattress I bought with a used bunkbed was 97% polyurethane foam.  I think that I am going to have to have to shop for mattresses. 

Foam Mattresses Bad for Asthma: The homes of 152 schoolchildren in Northern Norway with dust samples from 24 foam mattresses without covers, 68 foam mattresses with covers, and 24 spring mattresses were studied. Mite feces were found in 40% of dust from foam mattresses without covers and 26% from foam mattresses with covers and 12.5% from spring mattresses. Signs of dampness, age of mattress, cleaning of mattress, frequency of vacuuming the bedroom, mechanical ventilation, bedroom temperature and bedroom relative humidity were associated with the presence of mite feces. After adjustment for these, the risk of finding mite feces was four times higher in foam compared to spring mattresses (odds ratio 4.0), and eight times higher for the foam mattresses without cover (OR 8.1). House-dust mites and mattresses. Schei MA, Hessen JO, Lund E. University of California, Berkeley. Allergy. 2002 Jun;57(6):538-42

Foam Pillows Bad for Asthma; Feather Pillows Good: 486 children with 12 or more wheezing attacks in the past year were compared to 475 similar children without attacks, all living at the same address for at least 3 years. Severe wheeze was more common with non-feather bedding, especially foam pillows (odds ratio 2.78), and the ownership of furry pets now (1.51) and at birth (1.70). These estimates were family not making changes in the bedroom or pet because of allergy. If the foam pillow association is causal, it accounts for 53% of the severe asthma in the study. Parental smoking, use of gas for cooking, age of mattress, and mold growth in the child's bedroom were not significantly associated with wheezing. Home environment and severe asthma in adolescence: a population based case-control study. Strachan DP, Carey IM. St George's Medical School, London. BMJ. 1995 Oct 21;311(7012):1053-6.

Household Clemicals Linked to Early Asthma Signs: In a prospective 3.5 year follow-up study of 14,000 newborns, researchers found that in the 10% of families that used household chemicals, such as bleach, paint stripper and carpet cleaners most often, there children were twice as likely to suffer from wheezing as those who used the least. Prenatal as well as postnatal exposure appeared to play roles in increase asthma risk. The 11 most common chemicals were disinfectants (used by 87.4%), bleach (84.8%), carpet cleaner (35.8%), window cleaner (60.5%), dry cleaning fluid (5.4%), aerosols (71.7%), turpentine/white spirit (22.6%), air fresheners - spray, stick or aerosol (68%), paint stripper (5.5%) , paint or varnish (32.9%) and pesticides/insecticides (21.2%). 6.2% had a persistent wheeze from infancy; 70% had never experienced wheezing, 19% wheezed as babies but not when they were older, and 3.5% developed wheezing problems after the age of two-and-a-half. Other studies have shown people working as cleaners are at increased risk of developing asthma, and that toddlers exposed to fumes from solvents and cleaning products at home are also at high risk. Andrea Sherriff, University of Bristol, Thorax 12/04.  

Mattress Covers Didn’t Help Asthmatics in Small Study: The use of anti-allergic mattress covers in patients with asthma can result in a large reduction in the level of house dust mite allergen in dust samples. In a 1-year DB PC 30 patients with asthma and house dust mite allergy, the use of anti-allergic mattress covers resulted in significant reductions in Der p 1 concentrations in carpet-free bedrooms. However, in patients with moderate to severe asthma, airways hyperresponsiveness and clinical parameters were not affected by this effective allergen avoidance. Clinical evaluation of the effect of anti-allergic mattress covers in patients with moderate to severe asthma and house dust mite allergy: a randomised double blind placebo controlled study. Rijssenbeek-Nouwens LH, Oosting AJ, de Bruin-Weller MS, Bregman I, de Monchy JG, Postma DS. Hilversum, The Netherlands. Thorax. 2002 Sep;57(9):784-90.

Natural Gas Cooking, Foam Pillows and Damp House Linked in Very Large Study: In a cross-sectional study of 10 902 schoolchildren were recruited from Hong Kong, Beijing, and Guangzhou average age 10 with skin-prick testing, the prevalence of wheezing in the past 12 months was linked to the use of gas cooking (108% increased risk), foam pillow (94% increase), and house dampness (84% increased risk) were significant risk factors for "current wheezing". The use of cotton quilt (30% decreased risk), breast-feeding (21% decrease), and attendance of daycare (27% decrease) were protective factors against "current wheezing". Disparity of asthma prevalence in Chinese schoolchildren is due to differences in lifestyle factors. Wang HY, et al. Guangzhou Medical College, China. Zhonghua Er Ke Za Zhi 2006 Jan;44(1):41-5.

Natural Cooking Gas, Foam Pillows, and Acetaminophen Linked to Risk: In a study of 3089 children ages 2-6 living in Hong Kong, the prevalence of wheeze ever and current wheeze were 16.7% and 9.3%. Two environmental factors in the first year of life were associated with wheezing attacks within the last 12 months. They were the use of foam pillow (45% increased risk) and the use of gas as cooking fuel (68% increased risk). Frequent use of acetaminophen was also associated with wheezing attack in the past 12 months. Symptoms of asthma and atopic disorders in preschool children: prevalence and risk factors. Wong GW, et al. Prince of Wales Hospital, Chinese University of Hong Kong. . Clin Exp Allergy 2007 Feb;37(2):174-9.

Natural Gas Stoves Increase Asthma: S. Australia study 1339 preschoolers found positive association between nitrogen dioxide exposure from gas appliances and the prevalence of respiratory symptoms. Domestic nitrogen oxide exposure, urinary nitrate, and asthma prevalence in preschool children. Ciuk J, Volkmer RE, Edwards JW. Arch Environ Health 2001 Sep-Oct;56(5):433-8.

Natural Gas: Not Significant Though Nothing Else Was Either: In two cross-sectional ISAAC phase I and III surveys of 6,996 6- to 7-year-old children, positive associations were observed regarding exposure to molds, environmental tobacco smoke, cooking with gas and space heating with fossil fuels, but most associations were not statistically significant. Present pet ownership did not show positive associations with respiratory symptoms. However, ownership at different times in life revealed positive associations, particularly in regard to birds owned in the first year of life (PR 1.51, 95% CI 1.11-2.06, for sleep disturbance due to wheeze and PR 1.28, 95% CI 1.01-1.63, for wheeze during the last 12 months, respectively). Symptoms of asthma and the home environment. The ISAAC I and III cross-sectional surveys in Munster, Germany. Behrens T, et al. University of Munster, Germany. Int Arch Allergy Immunol 2005 May;137(1):53-61.

Natural Cooking Gas Link Not Found for Young Children: In a study of 3000 children participating in a birth cohort study on development of allergy and asthma, at 4 years of age, a sub-sample of 647 children provided blood samples for antibody testing. No relationship was found between gas cooking and any of the respiratory or allergy outcomes except nasal symptoms. Gas cooking, kitchen ventilation, and asthma, allergic symptoms and sensitization in young children--the PIAMA study. Willers SM, et al. Universiteit Utrecht, The Netherlands. Allergy 2006 May;61(5):563-8.

Gas Stove Link Not Found In NHANES Survey: Using data from the US Third National Health and Nutrition Examination Survey among 445 adults with asthma, nearly half had a gas stove in their home (47.1%). There was no significant association between gas stove use and FEV1 (mean change 146 ml), FVC (0 ml), or FEF25%-75% (357 ml). There was also no significant relation between gas stove use and the risk of self reported cough (OR 0.8), wheeze (OR 1.5), or other respiratory symptoms. Controlling for sociodemographic, smoking, housing, and geographic factors did not appreciably affect these results. Gas stove use and respiratory health among adults with asthma in NHANES III. Eisner MD, et al. University of California, San Francisco. . Occup Envir Med 2003 Oct;60(10):759-64.

Natural Cooking Gas: A Factor Though Less Than Mite Allergy: In a 4.6 year prostpective study of 156 male children and 168 female children without a prior diagnosis of asthma with an average age at follow-up of 13.4 yrs and 59% reaching puberty, the prevalence of bronchial hyperresponsiveness (BHR) at follow-up was similar among pre- and postpubertal male children (25.0% versus 29.2%),while BHR was more common among post- compared with prepubertal female children (33.1% versus 14.2%). The major determinant of BHR was a positive skin test to dust-mite antigen. BHR was also linked to exposure to gas cooking and the presence of exercise-induced bronchospasm. Risk factors for bronchial hyperresponsiveness in late childhood and early adolescence. Ernst P, et al. McGill University, Montreal. Eur Respir J 2002 Sep;20(3):635-9.

Natural Gas Stoves Increase Asthmatic Attacks: Adult women asthmatics had increased use of rescue bronchodilators during weeks using gas stoves. Nitrogen dioxide exposure from domestic gas cooking and airway response in asthmatic women. Ng TP, Seet CS, Tan WC, Foo SC. Thorax 2001 Aug;56(8):596-601.

Natural Gas Only Minor Problem in English Study: A sample of 1449 young adults born in Britain 3-9 March 1958, who have been followed from birth to ages 7, 11, 16, 23 and 33 years, were examined at home at age 34-35 years. FEV1 and FVC were measured before and 20 min after inhalation of 400 microg salbutamol, and skin prick tests performed with three allergen extracts (grass, Der p 1 and cat). An interview on respiratory symptoms and indoor environmental exposures was included. No association was found between gas cooking in childhood or adulthood and incidence or prognosis of asthma/wheeze, allergic sensitization or current severity of respiratory symptoms. Subjects who currently used gas for cooking had a significantly reduced FEV1 (- 70 ml) but not FVC (- 35 mL) compared with those who used electricity for cooking. Effects of exposure to gas cooking in childhood and adulthood on respiratory symptoms, allergic sensitization and lung function in young British adults. Moran SE, Strachan DP, Johnston ID, Anderson HR. Clin Exp Allergy 1999 Aug;29(8):1033-41

Natural Gas Doubles Lung Problems in Women: In a study of 1159 men and women in England from random sample of 15,000, women who reported they mainly used gas for cooking had a 107% increased risk of several asthma-like symptoms during the past 12 months including wheeze, 132% more waking with shortness of breath, and 160% more likely to have had asthma attacks. Gas cooking increased the risk of symptoms more in women who were atopic than in non-atopic women but the difference did not reach significance (p . 0.05). Women who used a gas stove or had an open gas fire had reduced lung function and increased airways obstruction. Association of respiratory symptoms and lung function in young adults with use of domestic gas appliances. Jarvis D, Chinn S, Luczynska C, Burney P. Lancet 1996 Feb 17;347(8999):426-31; Several other studies usually finding harmful effects from gas stoves. 

Smoking and Pets Common in Homes with Asthmatic Kids in Treatment: 38% homes of asthma kids have smokers vs. 35-45% national figure. 67% had at least one furry pet vs. 59% national average. J Pediatrics 7/02. Authors trivialized both issues by saying smokers are addicted and from lower class stressed families (and therefore can't quit), while pets were in middle-class homes and parents knew they were harmless to the child (sic).

Smoking: Second Hand Smoke a Major Cause of Asthma in Children: The Student Lung Health Survey in 136 Canadian schools found that among all 5-19 years old students, the prevalence of asthma was 13.0%, with the prevalence for males being slightly higher than for females: Odds Ratio (OR) was 1.17 for males compared to females. The proportion of asthma for students who smoked more than 11 cigarettes per day (OR = 1.41), were exposed to passive smoke in home (OR = 7.29), in car (OR = 4.71), and in school (OR = 4.24). Analysis of prevalence, triggers, risk factors and the related socio-economic effects of childhood asthma in the Student Lung Health Survey (SLHS) database, Canada 1996. Mo F, Robinson C, Choi BC, Li FC. Health Canada.  Int J Adolesc Med Health. 2003 Oct-Dec;15(4):349-58

Snoring Kids get More Asthma, Night Cough: Fifty-four of 516 2-5 year old boys (10.5%) and 48 of 458 girls (10.5%) were snorers on at least 4 nights per week. No trend association with age. Snoring and asthma (odds ratio, 2.03; p = 0.001). remained highly significant even among children without hay fever. Snoring and nocturnal cough (odds ratio, 3.68; p = 0 .001), and this was significant in both asthmatic and nonasthmatic patients when evaluated separately. Jennifer K. Peat, U Sydney, Chest 2003;124:587-593. Snoring also increases the risk of ADHD.

Steam-Heat Treatment and Bedroom Ventilation System Helped Asthma: The homes of 30 adult asthmatic were randomly allocated into 3 groups. In groups 1 and 2 mattresses and duvets were treated with hot air (110 degrees C), followed by steam and then heat again. All their carpets were steam cleaned. Group 2 also had a special ventilation system installed above each patient's bedroom. Active heat-steam treatment of homes caused a sustained reduction of Der p 1 (P =.003) and Der p 2 (P =.001). There was a 4-fold reduction in bronchial hyperreactivity at 9 months in group 1 and throughout the posttreatment period in group 2. Eradication of house dust mite from homes of atopic asthmatic subjects: a double-blind trial. Htut T, Higenbottam TW, Gill GW, Darwin R, Anderson PB, Syed N. University of Sheffield, Sheffield, UK. J Allergy Clin Immunol. 2001 Jan;107(1):55-60

Vacuum: High Energy Vacuum Cleaners Can Help Asthmatics: In a study of 60 dust mite-allergic asthmas, high energy vacuum cleaners did a better job removing allergens in the living-room carpet (p = 0.046), bedroom carpet (p = 0.003) and mattress (p = 0.013) and living-room sofa (p = 0.005), but only in the mattress sample using the standard cleaners (p = 0.014). Patients in the high-efficiency group showed improvements in peak expiratory flow rate (PEFR) (p = 0.004), FEV1 (p = 0.026) and bronchodilator usage (p = 0.005) after 12 months. When the cat-sensitive patients were analyzed separately, improvements in histamine PC20 (p = 0.039) were also seen. The effect of high-efficiency and standard vacuum-cleaners on mite, cat and dog allergen levels and clinical progress. Popplewell EJ, Innes VA, Lloyd-Hughes S, Jenkins EL, Khdir K, Bryant TN, Warner JO, Warner JA. University of Southampton, UK. Pediatr Allergy Immunol. 2000 Aug;11(3):142-8

Ventilation: Improved Ventilation Helped Asthmatics; High Energy Vacuum Helped: The homes of 40 house dust mite-sensitive asthmatic subjects were randomized to (1) mechanical ventilation and a high-efficiency vacuum cleaner (HEVC); (2) mechanical ventilation alone; (3) an HEVC alone; and (4) no intervention for 12 months. Homes with mechanical ventilation achieved significantly lower humidity levels than those without (P <.001), with an associated reduction of mite numbers (P <.05). The addition of a vacuum cleaner enhanced this effect. There was a trend for an improvement in histamine PC(20) (P =.085) in the patients whose homes were ventilated. A combined strategy of mite and mite allergen reduction in the control of mite-sensitive asthma. Warner JA, Frederick JM, Bryant TN, Weich C, Raw GJ, Hunter C, Stephen FR, McIntyre DA, Warner JO. University of Southampton, United Kingdom. J Allergy Clin Immunol. 2000 Jan;105(1 Pt 1):75-82