Allergin blir värre när det finns mer pollen.

Allergin is the definite singular form of allergi (allergy).

• en allergi = an allergy
• allergin = the allergy

Swedish often uses the definite form where English might use a more general noun, especially when talking about a known condition or situation.

Blir is the present tense of bli, which usually means become.

So Allergin blir värre literally means The allergy becomes worse.

This is a very common Swedish way to express change:

• det blir kallt = it gets cold
• hon blir trött = she gets tired

Because the sentence describes a change.

• är värre = is worse
• blir värre = becomes / gets worse

So blir shows that the allergy is worsening, not just describing its current state.

Värre is the comparative form of dålig or, in this kind of expression, the word used for worse.

Think of it like:

• bra → bättre = good → better
• dålig → sämre = bad → worse
• but Swedish also commonly says värre in the sense of worse, especially about symptoms, pain, problems, and conditions getting more severe

So blir värre means gets worse.

Here när means when.

It introduces a time-related clause:

• när det finns mer pollen = when there is more pollen

Depending on context, English might sometimes translate it as when or whenever.

Det finns is a very common Swedish expression meaning there is / there are.

Literally, finns comes from finnas, meaning exist / be found.

So:

• det finns pollen = there is pollen / there are pollen particles
• det finns mer pollen = there is more pollen

It works a lot like English there is / there are.

In a normal subordinate clause after när, Swedish keeps the subject before the verb:

• när det finns mer pollen

But in a main clause question, you would get inversion:

• Finns det mer pollen? = Is there more pollen?

So the word order changes depending on whether it is a statement or a question.

Actually, det in det finns often fills a similar role to English there in there is / there are.

So while det usually means it or that, in this expression it is part of the standard existential structure:

• det finns = there is / there are

You should learn det finns as a set phrase.

Swedish uses:

• mer for uncountable nouns = more
• fler for countable plural nouns = more

Pollen is treated as an uncountable substance here, so:

• mer pollen = more pollen

Compare:

• mer vatten = more water
• fler blommor = more flowers

Because pollen is being used as an uncountable noun in a general sense.

Just like in English, you usually do not need an article:

• more pollen
• mer pollen

If you were talking about a specific amount or type in another context, the structure could change, but here the bare noun is completely natural.

Because när det finns mer pollen is a subordinate clause.

In Swedish, subordinate clauses usually keep the order: subject + verb

• det finns

This is different from main clauses, where Swedish often has verb-second word order.

Compare:

• Main clause: Det finns mer pollen.
• Subordinate clause: ... när det finns mer pollen.

In both of these, the order stays natural because det is the subject and finns is the verb.

Yes, in many contexts learners may hear both värre and sämre, but värre is especially natural when talking about symptoms, pain, illness, or something becoming more severe.

Very roughly:

• sämre often means worse in a broader quality sense
• värre often suggests something becoming more intense, serious, or unpleasant

In this sentence, blir värre sounds very natural.

Yes, it is natural and idiomatic.

A Swedish speaker would understand it immediately as meaning that the allergy gets worse when pollen levels are higher.

A few similar natural sentences are:

• Min allergi blir värre på våren. = My allergy gets worse in the spring.
• Det blir värre när det är mycket pollen. = It gets worse when there is a lot of pollen.

So the original sentence is perfectly good Swedish.