​Since car air fresheners are essentially scented products, there is a strong likelihood that it will trigger asthma or allergic symptoms in individuals who are at an increased risk of such conditions. For those who already have allergies, exposure to car air fresheners especially those that are strongly scented can further worsen the symptoms of asthma and allergies. If the allergy is severe enough, it can lead to a life-threatening form of allergic reaction known as anaphylaxis. 
According to the same report, more than a third of individuals who have asthma reported worsening of their health conditions when exposed to air fresheners. And this is not only car air fresheners they are talking about. 
Volatile organic compounds have been shown to trigger allergy symptoms or even aggravate existing asthma and allergy conditions. Some of the major volatile organic compounds used in many car air fresheners today include alcohols,  formaldehyde, esters, petroleum distillates, and even limonene. 

So, are car air fresheners safe if you have allergies? The very simple answer is 'no!'

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Q: How often should I change my air freshener? 
A: Different manufacturers of car air fresheners have their respective duration of effectiveness, often depending on the type of scent delivery. For instance, those that come in can typically last longer than gel types while cardboard types of air fresheners will not usually last that very long. 
That being said, how often you should change your air freshener is dependent on the type of product that you're using. It also depends on a variety of products such as the humidity and temperature of your car interior. Increased cabin temperatures can quickly reduce the lifespan of air fresheners so you'll be changing your contraptions a lot more frequently. 
 

Q: How can I make my car air freshener last longer? 
A: Making your car air freshener last longer can be tricky, but it can be done. It all depends on the type of air freshener that 
you have. 
If you have a gel type, make sure never to open the lid or the product itself completely. Many car owners make the very simple,  but costly mistake of fully opening gel car air fresheners. This will only hasten the speed at which the gel will be used up. 
If you have a paper or cardboard type of air freshener, there really isn't much you can do to prolong its fragrance. What you can do, however, is to 'recharge' the cardboard by soaking it in your own favorite scent. This is more of reusing these types of air fresheners rather than prolonging their lifespan. 
If you have liquid air fresheners, the best way to lengthen their lifespan is by opening them only halfway, especially if you live in a rather cool environment. If there's an odor that you want to overpower, that's the time you can open it to its fullest. 

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