Apache 2.4 not closing connections

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http://serverfault.com – I have an Apache 2.4 webserver that runs on a micro instance of AWS with CentOS. The apache is running PHP with a prefork MPM module. The server is accessed via an Amazon load balancer which has a health check that tries to access index.php webpage of the server every 60 seconds. The problem is that after 1-2 days, Apache reaches somehow 256 opened processes with index.php page and gives the following error: [Sun Sep 08 02:13:00.356051 2013] [mpm_prefork:error] [pid 1201] (12)Cannot allocate memory: AH00159: fork: Unable to fork new process [Sun Sep 08 02:13:13.656694 2013] [mpm_prefork:erro (HowTos)