How does air pollution affect lungs?
In this blog post, we will discuss “How does air pollution affect lungs?” First, we will talk about the basics of air pollution. Then, the article will give a brief description of the lungs. In addition, it will emphasize the relation between air pollution and your lungs. In the end, the article will provide detailed solutions to improve air quality.
How does air pollution affect lungs?
Air pollution can affect your lungs severely. Respiration of dirty air can irritate your airways, lungs and whole respiratory system. Such irritation can cause you the following:
- Coughing
- Shortness of breath
- Wheezing
- Chest pain
- Asthma
- Sore throat
- Lung cancer
Let’s first learn about the basics of air pollution.
What is air pollution?
The introduction of harmful agents into the atmosphere is known as air pollution. The toxic agents are known as pollutants. These pollutants are derived from natural and human activities, and they can be chemical or biological.
Major pollutants present in the air are particulate matter (PM), ground-level ozone (O3), NO2, SO2, and CO. Air also has lead, allergens, bacteria, viruses and volatile organic compounds (VOCs).
Besides climate change, air pollution has become a significant environmental risk to human health.
What is Outdoor and Indoor pollution?
Air pollution in the outside environment is called outdoor pollution. The examples of outdoor pollution are as follows:
- Transportation
- Combustion of fossil fuel
- Cooking activities
- Energy generation and consumption
- Agriculture
- Improper Solid Waste Management
- Industries and factories
- Household pollution
- Celebrations
- Construction and demolition
Air pollution created inside any closed structure is called indoor pollution. The examples of indoor pollution are as follows:
- Household activities
- Perfumes and fresheners
- Cooking fuel/activities
- Cigarette
- Radon
- Poor ventilation
- Uncleaned furniture
- Pets
- Asbestos
There are two leading causes of air pollution- natural pollution (happened due to natural events like dust storms) and anthropogenic pollution (happened due to human activities like combustion of fossil fuels).
What is AQI, and what levels make it unhealthy?
WHO has set up the air quality guidelines to regulate the emissions; however, the United States Environmental Protection Agency (US EPA) has calculated its own AQI to safeguard the public health of Americans.
Environment Protection Agency (EPA) has determined the air quality index (AQI) to know the air quality you inhale. The table given below will show how the air quality index looks like.
| Air Quality Index (AQI) | Levels of Health Concern |
| (When the AQI is in this range) | (air quality conditions are) |
| 0 to 50 | Good |
| 51 to 100 | Moderate |
| 101 to 150 | Unhealthy for Sensitive group |
| 151 to 200 | Unhealthy |
| 201 to 300 | Very Unhealthy |
| 301 to 500 | Hazardous |
The primary purpose of AQI is to inform residents about the impact of local air quality on their health. The Environment Protection Agency calculates the AQI for five major pollutants: ground-level ozone, particulate matter, carbon monoxide, sulphur dioxide, and nitrogen dioxide.
The meaning of the Air quality index as mentioned earlier categories are as follows:
| Meaning |
| Air quality is considered satisfactory; air pollution is of little or no risk. |
| Air quality is acceptable; some pollutants present in the air might be dangerous for a minority of people who are very sensitive to air pollution. |
| Members of sensitive groups may get ill effects. However, the general public is likely not to get affected. |
| This air quality level can pose a threat to everyone. However, members of sensitive people may experience more severe effects. |
| Emergency level of air quality; Entire population might be affected |
| Health alert: Every person breathing such air quality may get serious health issues. |
Thus, it is clear that excluding the first two levels are relatively good to venture out and to do various outdoor activities. However, the last three levels will give you or other adverse effects by inhaling dirty air.
After getting a brief idea about air pollution, let’s move ahead to understand your lungs.
What are lungs, and why do you need them?
Lungs are two sponge-like organs located on either side, and it is in the thoracic cavity of the chest. They are a significant part of the respiratory system of your body. Do you know that your lungs function similar to sponges? The main job of those lungs is to absorb or take in or inhale all the fresh air and then to take out or exhale all the waste gases. The fresh air taken in is then used by other organs of the body for proper functioning. Thus, I also like to call them our very own trees of the body.
Along with the lungs, these are the body parts that make your respiratory system complete:
Nose: The nose takes in the air, and the nose hair and mucus lining filters it and moisten the air. Your nose also helps you to smell the things around you.
Mouth: The mouth also acts similar to the nose; it can supply air to the lungs. It is also known as the oral cavity.
Throat (Pharynx): It is a ring-like muscular tube in shape and helps carry liquid, food and air. It is located behind the nose and mouth, and it is connected to them.
Voice Box (Larynx): As the name says, this organ allows you to speak and permits air to the lungs. While allowing the air to pass into it, it blocks food and liquid from entering it.
Windpipe (trachea): It is a flexible tube essential for the entry of oxygen into your body. The main job of the windpipe is to carry air in and out of your lungs.
Airways (bronchi): Your airway also acts like a trachea. It carries air to and fro from your lungs. In addition, your airways are lined with the cells of mucus.
So speaking in a medical language, once inhaled through the nose or mouth, the diaphragm gets expanded to gather all the air needed, then it gets relaxed during exhalation of air. But, unfortunately, we inhale some unwanted agents too.
Let’s see in detail how these agents can affect your lungs.
Does Poor Air Quality affect your lungs?
Yes, poor air quality can affect your lungs. As mentioned earlier, airborne pollutants can irritate your airways and respiratory system to cause various respiratory diseases. The airborne pollutants which can cause your lungs trouble are as follows:
- Ground-level ozone
- Particulate Matter
- Sulphur dioxide
- Nitrogen dioxide
- Carbon monoxide
- Allergens
- Irritants
However, as per the British Lung Foundation, air pollution can affect everyone’s lungs, but it can worsen the health of those who already live with existing lung conditions; it can intensify the symptoms such as asthma attacks or COPD.
Medical research advises that brief and prolonged exposure to elevated levels of air pollution can cause detrimental effects on the lungs. Ample of evidence have observed that prolonged exposure to air pollution may lead to lung cancer and asthma. British Lung Foundation further said that extended exposure to air pollution could profoundly affect the lungs of children. Research conducted in California discovered that children living in highly polluted areas had a weaker immune system, resulting in asthmatic symptoms. So, along with children, let’s see whose lungs are sensitive to the polluted air.
Women and Children
The study published in the Journal of Allergy and Clinical Immunology of 2010 stated that they examined around 181 children with and without asthma in the California cities of Fresno and Palo Alto. Researchers selected Fresno city kids as this city is known for poor air quality, Palo Alto has comparatively lower air pollution.
The researchers found that exposure to high levels of air pollution had suppressed the immunity system’s regulatory T cells (Treg). As a result, it lowered the level of Treg function. These reduced levels of Treg cells were distinctly linked to asthma symptoms and decreased lung capacity.
The study also revealed that pregnant women smoking cigarettes caused a potential threat to the fetus development; the smoke can cause alterations in the molecules. In addition, tobacco smoke exposure during pregnancy causes interruptions in the lung development of the fetus. Researchers further analyzed that children at early stages and later teens fell for asthma or experienced asthmatic problems.
The research pointed out that sources such as vehicle exhaust, industrial emissions, polyaromatic hydrocarbon (PAH) from the combustion of fossil fuels were responsible for weakening the immune system. Thus, such weakening of the immune system directly corresponded to asthma generation among kids and adults.
Another research by Vanderbilt University and John Hopkins found that women were two times weaker lungs than men. This trend seems to continue till women reach menopause. They found that testosterone or male hormone cells stopped the swelling linked to asthma symptoms and hindered mucus production in the lungs.
People of Colour
Many epidemiologists have observed that the effects of air pollution don’t equally affect a region’s demography. For example, financially backwards people can get the heat of air pollution and selected racial and ethnic groups. The American Lung Association researched such visible differences and observed that some people of a racial or ethnic group have some distinct points for their lung conditions.
Firstly, they have a low socioeconomic position, and they cannot afford some of the appliances like an air purifier to improve the air quality. Secondly, the lack of awareness and education—the less educational background make them less aware of the basics of air pollution. Lastly, their settlements are closer to the major contributor of pollution—although there is much less to say here, their financial constraints make them settle in the affordable areas.
The latest assessment conducted by the EPA about particulate exposure revealed that non-white populations were more affected than the others. The most vulnerable people to the particulate pollution in the United States were the blacks. Asthma and the Allergy Foundation of America noted that black Americans are five times more at risk than white Americans to visit the emergency department due to asthma.
Older Adults
Older adults aged above 65 years are more susceptible to air pollution. This is because of their weak immune system to fight any diseases related to respiration and other organs. The American Lung Association has also found out that senior citizens with existing medical issues such as diabetes, asthma, or hypertension are at more risk of getting lung conditions. In addition, some medical researchers and doctors have noted that senior citizens living in highly polluted cities have more threat of lung cancer.
Another study by Simoni M. et al. discovered that poor indoor air quality could also lead to improper lung efficiencies among senior citizens than the younger counterparts. But, again, this is because the elders spend more time at home than the others.
Labours
Mathew Neidell of Columbia University examined the connection between labours and air pollution. He observed that workers working in mining and smelting areas, constructional sectors, automobile sector, and industries have more chances of impaired lung functions. He further suggested that improving the air quality regulations can improve work productivity and boost the economic sector.
Let’s just briefly see which of the main pollutants affect the lungs health.
Which main pollutants affect your lungs health?
Two main air pollutants affect the health of the lungs. Those are ground-level ozone smog and soot.
Ground-level ozone smog
Ground-level ozone smog is formed when Volatile Organic Compounds (VOCs) and noxious (NOx) gases are combined under the presence of sunlight. This smog is prevalent in high-level temperatures with no winds, and ozone levels increase in the evenings and minimum in the sunrise. The natural ozone in the atmospheric layer is beneficial for survival on the earth. However, artificial ground-level ozone is much more lethal.
VOCs are produced by the combustion of toxic chemicals, which are later vapourised and forms gas. NOx or noxious gases are formed when oxygen is added with nitrogen during the combustion process under high temperatures. Ground-level ozone smog is formed from combustion activities, vehicular activities and natural events such as wildfires.
Soot
The other type of air pollutant which does not get filtered through our nose and mouth is known as soot. It is also known as particulate matter and is measured in micron per metre cube. These particles are even tiny than the diameter of a hair strand or a sea sand particle.
These notorious tiny matters are present in liquid, gas or air droplets. Soot formation occurs due to anthropogenic activities like coal mining, power plants and natural phenomena such as volcanic ash from the eruption.
The soot is detected in the air during the formation of ground-level ozone. The conditions favourable for soot formation are dry and clear, and it is hazy in appearance.
These chemical and biological agents are so minute that, when inhaled, they can blend into your bloodstream and they can circulate with your oxygen and cause other health effects. They also age your lung skin. The other health effects due to air pollution are as follows:
However, there is no need to fear after reading this. I will give you detailed solutions to fight air pollution and to make it strong.
What can I do to protect myself from air pollution?
I will elaborate on some of the measures to protect yourself from air pollution.
Personal Hygiene
You can also fight the effects of air pollution by maintaining personal hygiene:
- You can cut your nails short to avoid the accumulation of dirt.
- You can also practice mouth gargling after coming home to free your mouth from unwanted guests.
- You can also brush twice a day to keep airborne infections at bay.
- You can also take steam therapy at home to cleanse your lungs.
Quit Smoking
Smoking cigarettes, hookahs (shisha), electric vape or any other types of cigars are not at all suitable for your health. It’s best to stop smoking. Active and passive smoking provides the direct entry of harmful pollutants into your body. This causes the risk to your respiration and other organs dependent on it.
Physical Exercise
Physical activities have many benefits to your body. It not only improves your stamina and immunity levels but also helps to increase your lung capacity. Yoga, sanas, swimming, running, cycling, brisk walking and dancing are some of the best forms of exercise to improve your health.
Breathing Exercise
The majority of people in the world don’t breathe properly. However, correct breathing techniques can keep illnesses due to air pollution at bay. A pranayama is a form of yoga asana which helps to improve breathing methods and lung functionality.
Some of the pranayama types that help you fight air pollution are Kapalbhati, Anulom Vilom, Chandra Nadi, Surya Nadi and Bhastrika. It is strongly recommended to learn this art of breath from a well-certified yoga trainer.
You can also increase your lung muscles by simply taking a deep breath and holding it for a count of 10. Then, slowly breathe out the air. If you can keep your breath for a longer time, then you have good lungs health.
Good diet
Always go for a well-balanced diet to remain fit and strong. Consistent eating of junk and fast food will increase the risk of obesity and other diseases. In addition, such people are more vulnerable to the adverse effects of air pollution. Instead, add more fresh veggies and fruits to your diet. Also, you can consume less meat to minimise carbon footprints.
You can also add anti-inflammatory foods as these types of food help to cleanse your lungs effectively. Anti-inflammatory foods which I recommend are:
Immunity booster drinks
You can do your immunity booster drinks by using readily available items in your kitchen. Immunity booster drinks help to cleanse your lungs. Some of the DIY immunity booster drinks are as follows:
- Take a glass of warm white milk. Add half a teaspoon of turmeric powder to it. Also, add two strands of saffron to it. Drink it at night before sleeping. You will get good sleep, and you will be relieved from chest congestion.
- Boil water by adding half cinnamon stick. Drink the spice-infused water once a day. Your sinuses would be cleared up.
- Buy an excellent green tea. Prepare a green tea by following the packet instructions. Have it whenever you feel irritation in your throat.
- Take some crushed 1 or 2 peppercorns, a half-inch of a blade of lemongrass and 1 inch of grated ginger and half a teaspoon of turmeric powder. Add everything in one glass of boiling water and reduce it to half a glass. Drink two tablespoons of this spice-infused warm water every after three hours. Adding honey is optional.
- You can also drink warm water and add honey if you want. Honey helps to relieve throat irritation.
Use Mask
Mask can prevent you from inhaling the polluted air. You would be able to work in contaminated areas without any fear of allergy infections and stress.
Masks I recommend
| Name | Why I recommend it |
| BASE CAMP CROSSDUST MASK | EAPI Filter Technology & Advanced NanotechnologyUser FriendlyMore Durable Six layers face maskFilter materials of the dust mask: Double-Layer Melt-blown with Electrostatic Adsorption Performance, Activated carbon, Non-woven Polypropylene |
| NIOSH approved N95 mask | Four layers mask Flexible nose pad, dual strapsDisposable maskEasy to breathe |
| BASE CAMP Reusable Cloth Face Mask | Three layers mask; 100% cotton Skin-friendly and breathablePocket to add additional filterReusable Mask |
How can I individually reduce air pollution?
You can always try your level best to minimise air pollution. The following are the methods to reduce air pollution at an individual level.
Plant Trees
Encourage tree plantation drives amongst the communities. You can be a green saviour of your region. Trees are the natural air purifiers of the environment, and it supplies us with clean air.
Municipal councils or the local authorities can develop urban forests to increase the green cover of the locality. They can also plan to create vertical green walls to improve oxygen levels of the region naturally. But don’t forget to plant only indigenous trees.
Human/ religious celebrations in eco-friendly ways
Nowadays, many couples celebrate gender (of their baby) revealing parties by bursting firecrackers. But, unfortunately, these firecrackers can transform into massive wildfires if they are burst near trees.
Thus, you can celebrate such celebrations by planting trees in your nearby barren land. Bursting firecrackers on any religious, festive or personal occasion are only going to harm your loved ones.
You can also celebrate Christmas by not cutting Christmas Trees. Instead, you can buy artificial ones and use them in the coming years too.
Discourage Plastic
Single-use plastic has been one of the major concerns in the environment. Plastic pollution indirectly contributes to the degradation of air quality. Plastic, when burned, releases various toxic gases such as dioxins into the air.
Encourage alternative use of the plastic product. For example, you can wear a cloth bag for shopping. You can also carry your travel cutlery set to a restaurant. Additionally, you can take your water bottle instead of purchasing a packaged drinking water bottle every time.
Use of public transportation
Increase the use of public transport to reduce air pollution and traffic congestion. You can also try carpooling or the cab sharing option. Then, you can also use bicycles instead of scooters. Lastly, you can always opt for walking a shorter distance.
Alternative use of fossil fuel
It would be best to try switching to other fuel alternatives. For example, don’t use wood for the fireplace. Instead, you can wear multiple layers of clothes for warmth. In this way, you can conserve energy. Additionally, you can also install solar panels on house roofs to convert solar energy into electricity.
You can also buy electric and hybrid vehicles to minimise tailpipe emissions. Finally, propose or discuss a plan with your mayor or any relevant local authority to promote your region’s renewable energy sector.
Reduction of personal carbon footprints
You can also keep a check on your carbon footprints. For example, you can delete unwanted photos, videos or emails from clouds and email boxes as and when possible. Unfortunately, this unwanted stuff takes a lot of energy, which indirectly contributes to air pollution.
You can also switch off the lights and electrical appliances of the vacant rooms. Try buying local products instead of imported ones. Be vocal about them. You can also discover your own country rather than travelling to different ones.
Create Awareness
Air pollution is unpredictable; air pollution in your vicinity can affect people living in Asia and vice-a-versa. However, you can create public awareness about air pollution amongst your community by teaching and interacting with your kids about the importance of good air.
Educate youth by telling them the ill effects of poor air quality. Then, let your country’s future know that they deserve a better place to live!
How can the local authorities improve the air quality of your region?
You can always observe the changes happening in your surrounding nature. If you experience bad air quality or any activities producing air pollution, you can always concern relevant local authorities or the mayor.
Here are some methods which local authorities can do.
Proper disposal of waste
Improper solid waste management of an area is an invitation to many diseases and infections. In addition, burning solid waste in public can cause air pollution, although such practices are rare in the US.
The wastes are mainly disposed of in the engineered landfills and recycled before final disposal to landfills and waste incinerators.
In addition, waste incinerators help to convert waste into energy.
However, many researchers have claimed that gasification technology or waste-to-energy technology may release harmful gaseous contaminants. Thus, it is necessary that waste collectors rigorously segregate the waste before sending it to waste incinerators.
The best solution is to minimise waste at a personal level. Statistically, 70% of the waste generated can be recycled effectively. Thus, strive to recycle your waste at a personal level as much as possible.
Reducing air pollution from agriculture
Stubble burning is still practised in some parts of the US. This event contributes to the degradation of air. Local authorities can help farmers convert this waste into cattle fodder or be transformed into pellets as fuel. In addition, farmers should stop the usage of harmful pesticides or fertilisers to decrease air pollution.
How can I reduce indoor air pollution?
Maintaining household or indoor hygiene
You can decrease poor air quality by adequately maintaining indoor hygiene. For example, you can frequently clean your cupboards and open decks to avoid an accumulation of dust particles. You can then vacuum your carpets and sofas at least twice a week.
Make sure your doggies and cats are well-groomed. Use chimneys and exhaust fans in kitchens. You can also install an exhaust fan in washrooms. Always make sure you are keeping your house well ventilated.
Keep doors and windows open when possible. Maintain the health of electric appliances such as heaters, humidifiers, AC, fans, air ducts and so on at regular intervals of the year. Avoid overuse of strong perfumes or strong scented candles indoors. You can additionally use indoor plants to make your indoors more lively and fresh.
Indoor plants
Indoor plants do the same work as outdoor ones- they help to purify indoor air naturally. These indoor plants are also easy to maintain.
The indoor plants which I recommend are as follows:
- Snake plant
- English Ivy
- Peace Lily
- Red-Edged Dracaena
- Boston Fern
- Spider Plant
- Weeping fig
- Bamboo Palm
- Rubber Plant
Air purifiers
Air purifiers help to improve indoor air quality. The risk of getting air pollution-related diseases decreases due to the use of air purifiers. Air purifiers having good HEPA filters do an adequate job of filtering pollutants.
Air Purifiers I recommend are as follows:
| Name | Why I recommend it |
| Blueair Pro XL Air Purifier | 99.7% of airborne pollutants such as mould, allergens, smoke particles and pollens are captured. |
| AIRMEGA 400S | Reduction of 99.97% of particulates of 0.3 microns. Also responsible for reducing more than 99% of VOCs, NH3 and CH3CHO.Washable and permanent pre-filters are available |
| Blueair Pure 121 | Includes 3-part filtration system. Captures 99% of common airborne PM 2.5 pollutants such as allergens, odours, smoke, mould spores, dust mites and pet dander.An activated carbon filter removes common odours caused by smoke, pets, and gases (VOCs) and smoke caused by wildfires. |
Humidifiers
The humidity levels in the office should be maintained between 30 and 50% in the office. This range helps to keep dust mites, mould and other allergens under control. In addition, when AC or heater makes the air in the room dry, humidifiers can maintain correct humidity levels.
Humidifiers I recommend
| Name | Why I recommend it |
| Amazon Basics Ultrasonic Cool Mist Humidifier | 4 litre of tank capacityAuto shut-off feature and 12 or 24-hour timer settingsIt can be used in both seasons |
| MOVTIP Humidifier | Portable Mini-humidifier500 ml capacity, still produces longer mistsEasy to operate |
| LEVOIT Humidifier | It can be used for both seasons6 litres of capacity Easy to clean and has a remote control |
Dehumidifiers
Dehumidifiers also work similarly to humidifiers. It controls the high levels of humidity in the office.
Dehumidifiers I recommend
| Name | Why I recommend it |
| Pro Breeze Dehumidifier | Can extract 50 pints of moisture a dayEnergy-saving auto-off systemBuilt-in Humidity sensor |
| HOmeLabs Dehumidifier | Eliminates moistness, odour controllerEasy to cleanRemovable water tank |
| TCL Portable Dehumidifier | 20 pints of moisture removed per dayIt can be operated at low temperatures tooEasy to maintain |
Other FAQs about Air Quality that you may be interested in.
How does air pollution affect food?
How does air pollution affect global warming?
How does air pollution affect the heart?
Conclusion
In this blog post, we discussed “How does air pollution affect lungs?” First, we talked about the basics of air pollution. Then, the article gave a brief description of the lungs. In addition, it emphasized the relation between air pollution and your lungs. In the end, the article provided detailed solutions to improve air quality.
Frequently Asked Questions (FAQs): How does air pollution affect the lungs?
What are the famous effects of air pollution on humans?
The famous effects of air pollution on humans are asthma, lungs disorders, Alzheimer’s and Parkinson’s disease, low birth weight, heart diseases and improper fetal growth.
How does air pollution affect the lungs?
Air pollutants present in the air can be harmful and toxic. When they are inhaled, they can irritate your lungs. This may cause chest congestion and irritation, coughing, wheezing and shortness of breath. Prolonged exposure to air pollution can cause lung diseases such as lung cancer.
Which of our organs is mainly affected by air pollution?
Your heart and lungs are most affected due to air pollution. The harmful effects of air pollution can give you breathing problems and can disturb the proper functioning of the heart and circulatory system. Air pollution can cause asthma, heart attacks and strokes.
Which pollutants can damage the lungs?
Many airborne pollutants can damage the lungs; they are as follows:
- Ozone
- Particulate matter
- Nitrogen oxides
- Sulphur oxides
Which type of air pollutant can notably cause lungs infection?
Exposure to elevated levels of particulate matter 2.5 can cause lung infection. In addition, short-term exposure to particulate pollution can develop acute lower respiratory infections in young children.
How do you clean your lungs from air pollution?
You can do the following things to clean your lungs from air pollution:
- Exercise a lot, practice physical and breathing exercise
- Go outdoors for hiking or enjoy nature trails to get more oxygen
- Plant more trees
- Have a healthy diet full of vegetables and fruits
- Avoid artificial scents and perfumes if you have allergic history.
References
Air pollution speeds up the ageing of the lungs and increases chronic lung disease risk. ScienceDaily. Available at: https://www.sciencedaily.com/releases/2019/07/190708201829.htm
Air pollution. Asthma and Allergy Foundation of America. Available at: https://www.aafa.org/air-pollution-smog-asthma/
Air Pollution. World Health Organization. Available at: https://www.who.int/health-topics/air-pollution#tab=tab_1
Asthma. World Health Organization. Available at: https://www.who.int/news-room/fact-sheets/detail/asthma
Disparities in the Impact of Air Pollution. American Lung Association. Available at: https://www.lung.org/clean-air/outdoors/who-is-at-risk/disparities
Effects of air pollution on the elderly. Omaha. Available at: https://emoha.com/blogs/health/effects-of-air-pollution-on-the-elderly
Neidell, M., 2017. Air pollution and worker productivity. IZA World of Labor. Available at: https://wol.iza.org/articles/air-pollution-and-worker-productivity/long
Simoni, M. et al., 2003. Indoor air pollution and respiratory health in the elderly. European Respiratory Journal, 21(Supplement 40).
The Links Between Air Pollution and Childhood Asthma. EPA. Available at: https://www.epa.gov/sciencematters/links-between-air-pollution-and-childhood-asthma
What causes asthma? American Lung Association. Available at: https://www.lung.org/lung-health-diseases/lung-disease-lookup/asthma/asthma-symptoms-causes-risk-factors/what-causes-asthma