The conjunctiva is a very thin, transparent mucous membrane that lines and protects the white part of the eyes (called the sclera) and the inner surface of the eyelids. Its main functions are the protection and lubrication of the other external eye structures against harmful agents, as well as immune defence. The most common harmful agents belong to one of three groups: allergens, microorganisms and toxic substances.
Allergic conjunctivitis is an inflammation of the conjunctiva triggered by allergens. The most obvious symptoms of allergic conjunctivitis are redness, pruritus (itching) and watery eyes.
These symptoms are very bothersome and, when intense, can considerably limit day-to-day activities and interfere with sleep: a study in children with allergic conjunctivitis found poorer sleep quality in both the children and their parents. That is why a correct diagnosis and appropriate treatment are important, not only to relieve the symptoms but also to prevent them from getting worse and to avoid complications.
In this article, we explain what allergic conjunctivitis is, how it differs from other types of conjunctivitis, how long it lasts, when you should see a doctor and what measures you can take to relieve and prevent the symptoms.
What is allergic conjunctivitis?
It is estimated that at least 20% of the population suffers from allergic conjunctivitis, according to a review published in the Revista Portuguesa de Imunoalergologia. This condition rarely occurs on its own: it is usually associated with other allergic diseases, especially allergic rhinitis.
Most people with allergic rhinitis also have allergic conjunctivitis and, in a study carried out in Portuguese hospitals, around 46% of patients with allergic conjunctivitis also had allergic rhinitis. The eyes and nose are exposed to the same airborne allergens (substances capable of triggering allergic reactions).
As a result, nasal symptoms may appear at the same time, such as:
- nasal pruritus (itching);
- runny and blocked nose;
- bouts of sneezing.
The conjunctiva of the eye and the nasal mucosa contain specific immune cells, ready to respond to substances identified as foreign or irritating. This response causes intense inflammation, with the production of secretions (tears or nasal discharge) in an attempt to eliminate the unwanted substance.
What causes allergic conjunctivitis?
As with other allergies, the triggers of allergic conjunctivitis are harmless to most people. In allergic people, the immune system overreacts to these substances and causes intense inflammation.
Research also suggests that environmental pollutants and dysfunction of the barrier that protects the eye surface may play a role in this inflammatory response.
The main triggers of allergic conjunctivitis are:
- pollen;
- dust and dust mites;
- pet hair;
- mildew and mould;
- tobacco smoke, swimming pool chlorine, perfumes and cosmetics;
- vehicle exhaust fumes and air pollution, among others.
Conjunctivitis can also be caused by infectious agents (infectious conjunctivitis) or by toxic agents, for example at work or in accidents involving chemicals (toxic conjunctivitis). These two types of conjunctivitis can be more serious if they are not diagnosed and treated promptly.
What are the symptoms of allergic conjunctivitis?
The symptoms of allergic conjunctivitis can vary in intensity and include:
- red eyes;
- burning eyes with intense pruritus (itching);
- heavy tearing;
- oedema (swelling) of the eyelids and of the conjunctiva;
- a gritty, “sandy” feeling in the eyes;
- “thickening” of the conjunctiva of the eyelids (mild papillary hypertrophy).
The symptoms usually affect both eyes and may not all be present at the same time. Tearing and discomfort can make vision seem “hazy”, but a persistent reduction in vision is a warning sign that needs medical assessment.
Sensitivity to light (photophobia) may also occur.
Seasonal or persistent allergic conjunctivitis
Like allergic rhinitis, allergic conjunctivitis can be seasonal, related to pollination. In this case, the symptoms are more frequent in the months of highest pollen exposure, particularly in spring and autumn.
When it lasts all year round or for longer periods, it is called persistent or perennial.
How is allergic conjunctivitis diagnosed?
Although allergic conjunctivitis can greatly disrupt quality of life and wellbeing, not least because it can drag on over time, it is important to distinguish it from other forms of conjunctivitis. Toxic conjunctivitis requires immediate and safe treatment, and infectious conjunctivitis requires a timely diagnosis and specific treatment.
The diagnosis should be made by a doctor and may require an examination by a specialist in ophthalmology or in allergy and clinical immunology, to ensure a more reliable and specific diagnosis.
The diagnosis of allergic conjunctivitis is based on examination and, above all, on the medical history and the presence of other associated allergic diseases, such as rhinitis and allergic asthma.
In persistent or hard-to-control cases, blood tests to identify specific antibodies and skin prick tests with allergens (“allergy tests”) may be needed. These tests help to identify the allergens responsible.
What is the difference between allergic and infectious conjunctivitis?
Infectious conjunctivitis is caused by viruses (in most cases) or by bacteria. It may start in one eye and, according to SNS 24 (the Portuguese health service line), it is often the person themselves who spreads it to the other eye. Unlike the allergic form, it is contagious.
Some symptoms of allergic conjunctivitis are shared with infectious conjunctivitis, such as red eyes, watery eyes and a gritty, “sandy” feeling in the eyes. Even so, there are important differences:
- Allergic conjunctivitis: there is often a history of allergy and, according to the American Academy of Ophthalmology, it usually causes intense itching and heavy tearing and affects both eyes; the eyelids may become puffy.
- Infectious conjunctivitis: in the bacterial form, the discharge tends to be thick and yellowish or greenish and can make the eyes stick together, especially on waking; in the viral form, the discharge is usually watery.
If you suspect infectious conjunctivitis, wash your hands often and do not share towels, pillows or make-up, as recommended by SNS 24 and the NHS, to reduce the risk of passing it on.
What is toxic conjunctivitis?
Toxic conjunctivitis is caused by direct contact with a toxic substance, for example an accidental chemical splash, industrial fumes or some cosmetics. It requires specialist, safe and rapid intervention, as it can become serious if left untreated.
If a chemical gets into your eyes, rinse them immediately with plenty of running water for at least 15 minutes and go to an emergency department without delay, as MedlinePlus first aid guidance advises.
Toxic conjunctivitis can also result from a reaction to eye drops, including the preservatives they contain. That is why, whenever possible, preservative-free, single-dose eye drops should be preferred.
Allergic eye reactions can have many origins, and the reaction mechanisms vary from person to person. For this reason, not all eye allergies are allergic conjunctivitis, even though they may share some symptoms.
How long does allergic conjunctivitis last?
There is no single answer. The duration depends on the type of allergens and how long they remain in the environment of the affected person, as well as on the presence of other associated allergic diseases, such as allergic asthma and rhinitis.
In seasonal allergic conjunctivitis, the symptoms can continue throughout the pollen season if exposure continues and there is no appropriate treatment. In general, according to SNS 24, conjunctivitis can last from one week to 15 days, but its duration depends greatly on what triggered it.
In chronic situations, where the allergens remain in the environment (dust and dust mites, pet hair or occupational exposure), it can persist for weeks or months, with periods when symptoms worsen and periods of relief.
When should you see a doctor?
See a doctor if the symptoms do not improve with the usual measures, come back often, are accompanied by thick yellowish discharge or last longer than one to two weeks, after which the American Academy of Ophthalmology advises seeing an ophthalmologist.
Warning signs: when to seek urgent help
According to the NHS and the American Academy of Ophthalmology, you should seek urgent medical assessment if you notice:
- severe eye pain;
- changes in vision (reduced or blurred vision);
- marked sensitivity to light (photophobia);
- very pronounced redness;
- a red, painful eye in someone who wears contact lenses;
- contact of the eyes with a chemical, or an eye injury.
If in doubt, you can contact your country’s health helpline (in Portugal, SNS 24: 808 24 24 24).
Allergic conjunctivitis treatment
Treatment should be adjusted to the severity of the symptoms. The aim is to stop or reduce the inflammatory cascade associated with the allergic response, relieve the symptoms and prevent the complications of chronic inflammation.
Treatment of allergic conjunctivitis covers four areas of intervention:
- preventive strategies, to reduce exposure to allergens;
- non-pharmacological measures;
- pharmacological treatment;
- immunotherapy.
Non-drug measures to relieve symptoms
- Wear sunglasses, to minimise contact with airborne allergens and reduce photophobia.
- Apply cold compresses, which can help to relieve itching and reduce redness and swelling of the conjunctiva.
- Use eye lubricants (artificial tears or saline solution) and keep good eyelid hygiene, which can strengthen the barrier function of the eye surface and help to remove allergens and inflammatory cells.
- Avoid scratching or rubbing your eyes, as this irritates them even more and increases swelling.
In general, these measures are not enough to resolve the symptoms, but they can help to ease them.
What medicines are used for allergic conjunctivitis?
Pharmacological treatment mainly uses medicines applied directly to the eyes (topical use), such as eye drops or ointments. This is the preferred approach, because of its good efficacy and limited systemic effects (acting throughout the body).
When allergic conjunctivitis is associated with allergic rhinitis, an oral antihistamine may be recommended, with systemic action.
The main categories of drugs used are:
- those that stop the allergic reaction cascade (antihistamines and mast cell stabilisers);
- those that reduce redness and swelling (vasoconstrictors);
- those that fight inflammation (non-steroidal anti-inflammatory drugs and corticosteroids), which should be used very judiciously.
Do not use eye drops on your own initiative for long periods. Eye drops containing corticosteroids should only be used with a prescription and medical supervision, because of the risk of serious side effects such as raised eye pressure and cataracts. Vasoconstrictors should only be used for short periods, because continued use can cause even worse redness when the effect wears off (rebound effect).
Allergen immunotherapy
Allergen immunotherapy (by injections under the skin or sublingual application) remains the only treatment option capable of modifying the natural course of allergic disease, but it requires specific clinical criteria.
It is recommended when moderate to severe symptoms that interfere with quality of life persist despite optimised treatment.
How to prevent allergic conjunctivitis?
Prevention is based on allergen avoidance measures (avoiding, minimising and removing) targeting the allergens identified during diagnostic testing. These measures are common to other allergic diseases, as in the prevention of allergic rhinitis.
Practical measures at home and in daily life
- Plan your outings during the pollen season by checking the pollen bulletins from the Rede Portuguesa de Aerobiologia (Portuguese Aerobiology Network), and keep windows closed on days with the highest pollen counts.
- Wear sunglasses outdoors and, when you get home, shower and change your clothes to wash off pollen.
- Keep your home clean, vacuum regularly with a vacuum cleaner with high-efficiency filtration (the NHS recommends a HEPA filter in your vacuum cleaner) and dust with a damp cloth.
- Wash bedding often in hot water, at least 54 °C, and use allergen-reducing covers on the bed and especially on pillows, to reduce exposure to dust mites.
- Cut down on carpets and rugs, which trap allergens such as pet dander, choosing wooden or tiled floors instead, and choose pillows made with hypoallergenic materials.
- If you are allergic to pet hair, keep animals away or at least keep them out of the bedroom.
- Avoid tobacco smoke, swimming pool chlorine and other irritants, as SNS 24 advises.
Prevention in the workplace
In the workplace, it is recommended to stay away from allergens or to take protective measures against them, such as wearing safety glasses when there is exposure to dust or fumes.
Indoor air purification
Whenever possible, you can use an air purifier with a certified HEPA filter. According to the US Environmental Protection Agency (EPA), a HEPA filter can theoretically remove at least 99.97% of airborne particles of 0.3 microns, such as dust, pollen and mould.
This measure can help to reduce exposure to allergens at home, where most people eat, sleep, relax and often work. In the US, people are estimated to spend around 90% of their time indoors.
Frequently asked questions about allergic conjunctivitis
Is allergic conjunctivitis contagious?
No. Allergic conjunctivitis results from an immune system reaction to allergens and is not passed from person to person. The contagious forms of conjunctivitis are the infectious ones, caused by viruses or bacteria.
Can you wear contact lenses with allergic conjunctivitis?
During flare-ups, people are usually advised not to wear contact lenses and to wear glasses instead; lenses should also be removed before using eye drops. If you wear contact lenses and have frequent symptoms, talk to your ophthalmologist about the most suitable type of lens and lens care.
Can allergic conjunctivitis be cured?
There is no definitive cure, but the symptoms can be well controlled with appropriate treatment and reduced exposure to allergens. In selected cases, immunotherapy can reduce sensitivity to specific allergens in the long term.
Does an air purifier help with allergic conjunctivitis?
A purifier with HEPA filtration helps to reduce airborne particles such as dust, pollen and mould spores. It is a measure that complements medical treatment and is most useful in the bedroom and in the rooms where you spend the most time.
Allergic conjunctivitis: the key points
Besides allergic conjunctivitis, there are other, more complex eye allergies that are beyond the scope of this article. Some can be serious if they are not correctly diagnosed and treated, so an assessment by an ophthalmologist is advisable if symptoms persist.
Medical assessment is especially important for non-seasonal eye allergies and for seasonal ones that get significantly worse, as some conditions can progress quickly and need timely intervention (see the warning signs above).
To prevent symptoms and protect eye health in the long term, any measure that reduces contact with allergens is useful, and many of these allergens are found inside our homes: cleaning and frequent washing of bedding, fewer dust-collecting textiles, less contact with animals in the bedroom and, where possible, indoor air purification with HEPA equipment.
If you want to improve the air quality in your home, discover the FilterQueen Defender air purifier or talk to the Filtrarte team for a recommendation suited to your space.
References
- Mota D, Vasconcelos MJ, Moreira A, Delgado L, Palmares J, Silva D. Alergia ocular: Da abordagem clínica ao tratamento. Rev Port Imunoalergologia 2024; 32 (1): 9-27. https://www.spaic.pt/client_files/rpia_artigos/artigo-de-reviso(17).pdf
- Marback PMF, Freitas D, Paranhos Junior A, Belfort Júnior R. Aspectos clínicos e epidemiológicos da conjuntivite alérgica em serviço de referência. Arq Bras Oftalmol. 2007;70(2):312-6. https://doi.org/10.1590/S0004-27492007000200022
- Leonardi A, Piliego F, Castegnaro A, Lazzarini D, La Gloria Valerio A, Mattana P, et al. Allergic conjunctivitis: a cross-sectional study. Clin Exp Allergy. 2015;45(6):1118-25. https://doi.org/10.1111/cea.12536
- Palmares J, Delgado L, Cidade M, Quadrado MJ, Filipe HP. Allergic conjunctivitis: a national cross-sectional study of clinical characteristics and quality of life. Eur J Ophthalmol. 2010;20(2):257-64. https://doi.org/10.1177/112067211002000201
- Li J, Zhang SY, Fan Z, Liu R, Jin L, Liang L. Impaired sleep quality in children with allergic conjunctivitis and their parents. Eye (Lond). 2023;37(8):1558-65. https://doi.org/10.1038/s41433-022-02182-4
- SNS 24. Conjuntivite. Accessed September 2026. https://www.sns24.gov.pt/pt/tema/doencas/doencas-oculares/conjuntivite
- American Academy of Ophthalmology. Conjunctivitis: What Is Pink Eye?. Accessed September 2026. https://www.aao.org/eye-health/diseases/pink-eye-conjunctivitis
- American Academy of Ophthalmology. Eye Allergies: Why Are My Eyes Itchy?. Accessed September 2026. https://www.aao.org/eye-health/diseases/allergies
- American Academy of Ophthalmology. Redness-Relieving Eye Drops. Accessed September 2026. https://www.aao.org/eye-health/treatments/redness-relieving-eye-drops
- NHS. Conjunctivitis. Accessed September 2026. https://www.nhs.uk/conditions/conjunctivitis/
- NHS. Hay fever. Accessed September 2026. https://www.nhs.uk/conditions/hay-fever/
- MedlinePlus, U.S. National Library of Medicine. Eye emergencies. Accessed September 2026. https://medlineplus.gov/ency/article/000054.htm
- MedlinePlus, U.S. National Library of Medicine. Allergic conjunctivitis. Accessed September 2026. https://medlineplus.gov/ency/article/001031.htm
- U.S. Environmental Protection Agency. What is a HEPA filter?. Accessed September 2026. https://www.epa.gov/indoor-air-quality-iaq/what-hepa-filter
- U.S. Environmental Protection Agency. Indoor Air Quality (Report on the Environment). Accessed September 2026. https://www.epa.gov/report-environment/indoor-air-quality
By the Filtrarte Expert Team
The information in this article is not intended to recommend any treatment for symptoms or diseases. Consult your doctor for specialist advice.
