Allergic sensitisation to house dust mite-associated aeroallergens is common in people with asthma and exposure to house dust can act as a trigger in sensitised asthmatic individuals. Physical (for example mattress covers, vacuum cleaning, heating, ventilation, freezing, washing, air filtration and ionisers) and chemical (acaricides) measures to reduce house dust mite (HDM) aeroallergen levels and so reduce exposure have been advocated but there has been uncertainty as to whether the currently available physical and chemical measures, alone or in conjunction, can reduce the exposure levels sufficiently to allow a clinically relevant effect to be apparent.

Animal allergens, particularly from cats and dogs, are potent provokers of asthma symptoms. The reported effects of removal of pets from homes are paradoxical, with either no benefit for asthma³³²,³³³ or a potential for continued high exposure to induce a degree of tolerance.³³⁴ In homes where there is no cat but still detectable cat allergen, there may be a benefit from introducing additional avoidance measures such as high-efficiency vacuum cleaners for patients allergic to cats, although there is insufficient evidence on which to base a recommendation.³³¹

Direct or passive exposure to cigarette smoke adversely affects quality of life, lung function, need for rescue medications for acute episodes of asthma and long-term control with ICS.^336-339

In children with asthma, exposure to environmental tobacco smoke is associated with worsening asthma symptoms.³⁴⁰

Uptake of smoking in teenagers increases the risks of persisting asthma.

Challenge studies demonstrate that various pollutants can enhance the response to allergen inhalation in patients with asthma.³⁴⁴, ³⁴⁵ Time-series and other observational studies suggest that air pollution may provoke acute asthma attacks or aggravate existing chronic asthma although the effects are very much less than in those with infection or allergen exposure.³⁴⁶, ³⁴⁷ Increased asthma symptoms in young children (mean age ≤9) have been linked, in observational studies, to exposure to air pollutants, including particulates, nitrogen dioxide, sulphur dioxide and ozone.³⁴⁰ Much less attention has been focused on indoor pollutants in relation to asthma and more work is required.³⁴⁸, ³⁴⁹

Information on current levels of air pollution, recommended actions and health advice is available from The Daily Air Quality Index (available at www.uk-air.defra.gov.uk).

Increasing dietary sodium has been implicated in the geographical variations in asthma mortality and high sodium intake is associated with increased bronchial hyper-responsiveness.^350-352 A systematic review of intervention studies reducing salt intake identified only minimal effects and concluded that dietary salt reduction could not be recommended in the management of asthma.³⁵³ Low magnesium intake has been associated with a higher prevalence of asthma with increasing intake resulting in reduced bronchial hyper-responsiveness and higher lung function.³⁵⁴ Magnesium plays a beneficial role in the treatment of asthma through bronchial smooth muscle relaxation, leading to the use of intravenous or inhaled preparations of magnesium sulphate for acute asthma attacks.³⁵⁵ Studies of oral supplementation are limited and more trials are required.^356-358

In vitro studies suggest that supplementing the diet with n-3PUFAs, which are most commonly found in fish oils, might reduce the inflammation associated with asthma.³⁵⁹, ³⁶⁰ Results from observational studies are inconsistent and a Cochrane review of nine RCTs concluded that there was insufficient evidence to recommend fish oil supplementation for the treatment of asthma.³⁶¹

Observational studies have reported that low intakes of vitamin C, vitamin E and selenium are associated with a higher prevalence of asthma.³⁰³ Intervention studies suggest that neither supplementation with vitamin C, vitamin E nor selenium is associated with clinical benefits in people with asthma.^362-364 Observational studies in both adults and children have also consistently shown that a high intake of fresh fruit and vegetable is associated with less asthma and better pulmonary function.^365-371 No intervention studies evaluating the intake of fruit or vegetables and their effects on asthma have been reported.

Further research is required on whether the effects of vitamin D supplementation are confined to people with lower baseline vitamin D status, and into the effects in children, and in people with frequent severe asthma attacks.

Although evidence is limited, these studies show that dietary and weight-loss interventions are feasible in overweight or obese adults and children with asthma and that they may improve asthma control, lung function and inflammation, although weight loss of greater than 10% may be necessary to achieve benefit.

In the absence of evidence of benefit, it is not possible to recommend the use of probiotics in the management of asthma.

A number of large studies have concluded that high vaccination coverage has no significant impact on any allergic outcome or asthma. There is a suggestion that the higher the vaccine coverage the greater the possibility that there is a degree of protection against the development of allergy in the first years of life.^382-385

Behavioural programmes centred on breathing exercises and dysfunctional breathing reduction techniques (including physiotherapist-delivered breathing programmes such as the Papworth method, and the Buteyko method) can lead to modest improvements in asthma symptoms and quality of life, and reduce bronchodilator requirement in adults with asthma, although have little effect on lung function or airway inflammation.^399-402

There is currently insufficient evidence on breathing exercises or yoga in children and adolescents aged 12 and under on which to base a recommendation.⁴⁰⁴, ⁴⁰⁶

1. 303. Tricon S, Willers S, Smit HA, Burney PG, Devereux G, Frew AJ, et al. Nutrition and allergic disease. Clin Exp Allergy Rev 2006;6(5):117-88.
2. 331. Leas BF, D'Anci KE, Apter AJ, Bryant-Stephens T, Lynch MP, Kaczmarek JL, et al. Effectiveness of Indoor Allergen Reduction in the Management of Asthma: A Systematic Review. J Allergy Clin Immunol 2018;13:13.
3. 332. Wood RA, Chapman MD, Adkinson NF Jr, Eggleston PA. The effect of cat removal on allergen content in household-dust samples. J Allergy Clin Immunol 1989;83(4):730-4.
4. 333. Wood RA, Johnson EF, Van Natta ML, Chen PH, Eggleston PA. A placebo-controlled trial of a HEPA air cleaner in the treatment of cat allergy. Am J Respir Crit Care Med 1998;158(1):115-20.
5. 334. Platts-Mills T, Vaughan J, Squillace S, Woodfolk J, Sporik R. Sensitisation, asthma, and a modified Th2 response in children exposed to cat allergen: a population-based cross-sectional study. Lancet 2001;357(9258):752-6.
6. 336. Chalmers GW, MacLeod KJ, Little SA, Thomson LJ, McSharry CP, Thomson NC. Influence of cigarette smoking on inhaled corticosteroid treatment in mild asthma. Thorax 2002;57(3):226-30.
7. 337. Ehrlich R, Jordaan E, Du Toit D, Potter P, Volmink J, Zwarenstein M, et al. Household smoking and bronchial hyperresponsiveness in children with asthma. J Asthma 2001;38(3):239-51.
8. 338. Gallefoss F, Bakke PS. Does smoking affect the outcome of patient education and self-management in asthmatics? Patient Educ Couns 2003;49(1):91-7.
9. 339. Mannino DM, Homa DM, Redd SC. Involuntary smoking and asthma severity in children: Data from the Third National Health and Nutrition Examination Survey. Chest 2002;122(2):409-15.
10. 340. Dick S, Doust E, Cowie H, Ayres JG, Turner S. Associations between environmental exposures and asthma control and exacerbations in young children: a systematic review. BMJ Open 2014;4(2):e003827.
11. 344. Devalia JL, Rusznak C, Herdman MJ, Trigg CJ, Tarraf H, Davies RJ. Effect of nitrogen dioxide and sulphur dioxide on airway response of mild asthmatic patients to allergen inhalation. Lancet 1994;344(8938):1668-71.
12. 345. Molfino NA, Wright SC, Katz I, Tarlo S, Silverman F, McClean PA, et al. Effect of low concentrations of ozone on inhaled allergen responses in asthmatic subjects. Lancet 1991;338(8761):199-203.
13. 346. Department of Health, Committee on the Medical Effects of Air Pollutants. Asthma and outdoor air pollution. London: HMSO; 1995. [cited
14. 347. Lin M, Chen Y, Burnett RT, Villeneuve PJ, Krewski D. Effect of short-term exposure to gaseous pollution on asthma hospitalisation in children: a bi-directional case-crossover analysis. J Epidemiol Community Health 2003;57(1):50-5.
15. 348. Norbäck D, Björnsson E, Janson C, Widstrom J, Boman G. Asthmatic symptoms and volatile organic compounds, formaldehyde, and carbon dioxide in dwellings. Occup Environ Med 1995;52(6):388-95.
16. 349. Tunnicliffe WS, Burge PS, Ayres JG. Effect of domestic concentrations of nitrogen dioxide on airway responses to inhaled allergen in asthmatic patients. Lancet 1994;344(8939-40):1733-6.
17. 350. Burney P. A diet rich in sodium may potentiate asthma. Epidemiologic evidence for a new hypothesis. Chest 1987;91(6 Suppl):143S-8S.
18. 351. Burney PG. The causes of asthma--does salt potentiate bronchial activity? Discussion paper. J R Soc Med 1987;80(6):364-7.
19. 352. Mickleborough TD, Lindley MR, Ray S. Dietary salt, airway inflammation, and diffusion capacity in exercise-induced asthma. Med Sci Sports Exerc 2005;37(6):904-14.
20. 353. Ardern KD, Ram FS. Dietary salt reduction or exclusion for allergic asthma (Cochrane Review). In: The Cochrane Library, 2001.
21. 354. Britton J, Pavord I, Richards K, Wisniewski A, Knox A, Lewis S, et al. Dietary magnesium, lung function, wheezing, and airway hyperreactivity in a random adult population sample. Lancet 1994;344(8919):357-62.
22. 355. Blitz M, Blitz S, Beasely R, Diner BM, Hughes R, Knopp JA, et al. Inhaled magnesium sulfate in the treatment of acute asthma (Cochrane Review). In: The Cochrane Library, 2005.
23. 356. Bede O, Suranyi A, Pinter K, Szlavik M, Gyurkovits K. Urinary magnesium excretion in asthmatic children receiving magnesium supplementation: a randomized, placebo-controlled, double-blind study. Magnes Res 2003;16(4):262-70.
24. 357. Fogarty A, Lewis SA, Scrivener SL, Antoniak M, Pacey S, Pringle M, et al. Oral magnesium and vitamin C supplements in asthma: a parallel group randomized placebo-controlled trial. Clin Exp Allergy 2003;33(10):1355-9.
25. 358. Hill J. Magnesium and airway reactivity. Clin Sci (Lon) 1998;95(2):111-2.
26. 359. Prescott SL, Calder PC. N-3 polyunsaturated fatty acids and allergic disease. Curr Opin Clin Nutr Metab Care 2004;7(2):123-9.
27. 360. Stephensen CB. Fish oil and inflammatory disease: is asthma the next target for n-3 fatty acid supplements? Nutr Rev 2004;62(12):486-9.
28. 361. Thien FC, De Luca S, Woods RK, Abramson MJ. Dietary marine fatty acids (fish oil) for asthma (Cochrane Review). In: The Cochrane Library, 2002.
29. 362. Allam MF, Lucane RA. Selenium supplementation for asthma (Cochrane Review). In: The Cochrane Library, 2004.
30. 363. Pearson PJ, Lewis SA, Britton J, Fogarty A. Vitamin E supplements in asthma: a parallel group randomised placebo controlled trial. Thorax 2004;59(8):652-6.
31. 364. Ram FS, Rowe BH, Kaur B. Vitamin C supplementation for asthma (Cochrane Review). In: The Cochrane Library, 2004.
32. 365. Butland BK, Strachan DP, Anderson HR. Fresh fruit intake and asthma symptoms in young British adults: confounding or effect modification by smoking? Eur Respir J 1999;13(4):744-50.
33. 366. Carey IM, Strachan DP, Cook DG. Effects of changes in fresh fruit consumption on ventilatory function in healthy British adults. Am J Respir Crit Care Med 1998;158(3):728-33.
34. 367. Cook DG, Carey IM, Whincup PH, Papacosta O, Chirico S, Bruckdorfer KR, et al. Effect of fresh fruit consumption on lung function and wheeze in children. Thorax 1997;52(7):628-33.
35. 368. Ellwood P, Asher MI, Bjorksten B, Burr M, Pearce N, Robertson CF. Diet and asthma, allergic rhinoconjunctivitis and atopic eczema symptom prevalence: an ecological analysis of the International Study of Asthma and Allergies in Childhood (ISAAC) data. ISAAC Phase One Study Group. Eur Respir J 2001;17(3):436-43.
36. 369. Gilliland FD, Berhane KT, Li YF, Gauderman WJ, McConnell R, Peters J. Children's lung function and antioxidant vitamin, fruit, juice, and vegetable intake. Am J Epidemiol 2003;158(6):576-84.
37. 370. Heinrich J, Holscher B, Bolte G, Winkler G. Allergic sensitization and diet: ecological analysis in selected European cities. Eur Respir J 2001;17(3):395-402.
38. 371. Strachan DP, Cox BD, Erzinclioglu SW, Walters DE, Whichelow MJ. Ventilatory function and winter fresh fruit consumption in a random sample of British adults. Thorax 1991;46(9):624-9.
39. 382. Gruber C, Illi S, Lau S, Nickel R, Forster J, Kamin W, et al. Transient suppression of atopy in early childhood is associated with high vaccination coverage. Pediatrics 2003;111(3):e282-8.
40. 383. Gruber C, Meinlschmidt G, Bergmann R, Wahn U, Stark K. Is early BCG vaccination associated with less atopic disease? An epidemiological study in German preschool children with different ethnic backgrounds. Pediatr Allergy Immunol 2002;13(3):177-81.
41. 384. Henderson J, North K, Griffiths M, Harvey I, Golding J. Pertussis vaccination and wheezing illnesses in young children: prospective cohort study. The Longitudinal Study of Pregnancy and Childhood Team. British Medical Journal 1999;318(7192):1173-6.
42. 385. Nilsson L, Kjellman NI, Bjorksten B. A randomized controlled trial of the effect of pertussis vaccines on atopic disease. Arch Pediatr Adolesc Med 1998;152(8):734-8.
43. 399. Bruton A, Lee A, Yardley L, Raftery J, Arden-Close E, Kirby S, et al. Physiotherapy breathing retraining for asthma: a randomised controlled trial. Lancet Respir Med 2018;6(1):19-28.
44. 400. O'Connor E, Patnode CD, Burda BU, Buckley DI, Whitlock EP. Breathing Exercises and/or Retraining Techniques in the Treatment of Asthma: Comparative Effectiveness. Rockville (MD): Agency for Healthcare Research and Quality (US); 2012. (Report number 12-EHC092-EF). [cited 10 Jul 2014] Available from http://effectivehealthcare.ahrq.gov/search-for-guides-reviews-and-reports/?pageaction=displayproduct&productID=1251
45. 401. Freitas Diana A, Holloway Elizabeth A, Bruno Selma S, Chaves Gabriela SS, Fregonezi Guilherme AF, Mendonça Karla MPP. Breathing exercises for adults with asthma. Cochrane Database of Systematic Reviews 2013: Issue 10.
46. 402. Prem V, Sahoo RC, Adhikari P. Comparison of the effects of Buteyko and pranayama breathing techniques on quality of life in patients with asthma - a randomized controlled trial. Clin Rehabil 2013;27(2):133-41.
47. 404. Yang Z-Y, Zhong H-B, Mao C, Yuan J-Q, Huang Y-F, Wu X-Y, et al. Yoga for asthma. Cochrane Database of Systematic Reviews 2016: Issue 4.
48. 406. Macêdo Thalita MF, Freitas Diana A, Chaves Gabriela SS, Holloway Elizabeth A, Mendonça Karla MPP. Breathing exercises for children with asthma. Cochrane Database of Systematic Reviews 2016: Issue 4.
49. 415. Panton J, Barley EA. Family therapy for asthma in children (Cochrane Review). In: The Cochrane Library, 2000.